Hi,
 Please find below code working fine for me...points count will be added into polygon layer ...updated into given filed.....
using System;
using System.Drawing;
using System.Runtime.InteropServices;
namespace CountPoints
{
    /// <summary>
    /// Summary description for CountAllPoints.
    /// </summary>
    [Guid("96ac8c8b-1a7c-457d-aa69-dcdcba6034f2")]
    [ClassInterface(ClassInterfaceType.None)]
    [ProgId("CountPoints.CountAllPoints")]
    public sealed class CountAllPoints : ESRI.ArcGIS.ADF.BaseClasses.BaseCommand
    {
        #region COM Registration Function(s)
        [ComRegisterFunction()]
        [ComVisible(false)]
        static void RegisterFunction(Type registerType)
        {
            // Required for ArcGIS Component Category Registrar support
            ArcGISCategoryRegistration(registerType);
            //
            // TODO: Add any COM registration code here
            //
        }
        [ComUnregisterFunction()]
        [ComVisible(false)]
        static void UnregisterFunction(Type registerType)
        {
            // Required for ArcGIS Component Category Registrar support
            ArcGISCategoryUnregistration(registerType);
            //
            // TODO: Add any COM unregistration code here
            //
        }
        #region ArcGIS Component Category Registrar generated code
        /// <summary>
        /// Required method for ArcGIS Component Category registration -
        /// Do not modify the contents of this method with the code editor.
        /// </summary>
        private static void ArcGISCategoryRegistration(Type registerType)
        {
            string regKey = string.Format("HKEY_CLASSES_ROOT\\CLSID\\{{{0}}}", registerType.GUID);
            ESRI.ArcGIS.ADF.CATIDs.MxCommands.Register(regKey);
        }
        /// <summary>
        /// Required method for ArcGIS Component Category unregistration -
        /// Do not modify the contents of this method with the code editor.
        /// </summary>
        private static void ArcGISCategoryUnregistration(Type registerType)
        {
            string regKey = string.Format("HKEY_CLASSES_ROOT\\CLSID\\{{{0}}}", registerType.GUID);
            ESRI.ArcGIS.ADF.CATIDs.MxCommands.Unregister(regKey);
        }
        #endregion
        #endregion
        private ESRI.ArcGIS.Framework.IApplication m_application;
        public CountAllPoints()
        {
            //
            // TODO: Define values for the public properties
            //
            base.m_category = "Spatial Analsis"; //localizable text
            base.m_caption = "Spatial Analsis";  //localizable text
            base.m_message = "Counts the point inside the polygon";  //localizable text 
            base.m_toolTip = "Counts the point inside the polygon";  //localizable text 
            base.m_name = "Spatial_Points_Inside_Polygon";   //unique id, non-localizable (e.g. "MyCategory_ArcMapCommand")
            try
            {
                //
                // TODO: change bitmap name if necessary
                //
                string bitmapResourceName = GetType().Name + ".gif";
                base.m_bitmap = new Bitmap(GetType(), bitmapResourceName);
            }
            catch (Exception ex)
            {
                System.Diagnostics.Trace.WriteLine(ex.Message, "Invalid Bitmap");
            }
        }
        #region Overriden Class Methods
        /// <summary>
        /// Occurs when this command is created
        /// </summary>
        /// <param name="hook">Instance of the application</param>
        public override void OnCreate(object hook)
        {
            if (hook == null)
                return;
            m_application = hook as ESRI.ArcGIS.Framework.IApplication;
            //Disable if it is not ArcMap
            if (hook is ESRI.ArcGIS.ArcMapUI.IMxApplication)
                base.m_enabled = true;
            else
                base.m_enabled = false;
            // TODO:  Add other initialization code
        }
        /// <summary>
        /// Occurs when this command is clicked
        /// </summary>
        public override void OnClick()
        {
            // TODO: Add CountAllPoints.OnClick implementation
            ESRI.ArcGIS.ArcMapUI.IMxDocument pMxDoc = m_application.Document as ESRI.ArcGIS.ArcMapUI.IMxDocument;
            ESRI.ArcGIS.Carto.IFeatureLayer pPolygonLayer = pMxDoc.FocusMap.get_Layer(1) as ESRI.ArcGIS.Carto.IFeatureLayer;
            ESRI.ArcGIS.Carto.IFeatureLayer pPointLayer = pMxDoc.ActiveView.FocusMap.get_Layer(0) as ESRI.ArcGIS.Carto.IFeatureLayer;
            ESRI.ArcGIS.Editor.IEditor pEditor;
            ESRI.ArcGIS.esriSystem.UID pUID = new ESRI.ArcGIS.esriSystem.UIDClass();
            pUID.Value = "esricore.Editor";
            pEditor = m_application.FindExtensionByCLSID(pUID) as ESRI.ArcGIS.Editor.IEditor;
            if(pEditor.EditState == ESRI.ArcGIS.Editor.esriEditState.esriStateEditing )
            {
                pEditor.StartOperation();
                GetAllPoitsInsidePolygon(pPolygonLayer.FeatureClass, pPointLayer.FeatureClass, ESRI.ArcGIS.Geodatabase.esriSpatialRelEnum.esriSpatialRelContains, "TVSID");//TVSID is filed name whihc should be in the poly layer
                pEditor.StopOperation("PolygonDensity");
            }
            else 
            {
              System.Windows.Forms.MessageBox.Show("Not in edit session");
            }
        }
        private void GetAllPoitsInsidePolygon(ESRI.ArcGIS.Geodatabase.IFeatureClass pFeatureClass1,ESRI.ArcGIS.Geodatabase.IFeatureClass pFeatClass2,ESRI.ArcGIS.Geodatabase.esriSpatialRelEnum lSpatialRel,String  strFeatFld)
        {
              ESRI.ArcGIS.Geodatabase.IFeatureCursor pFeatCursor = pFeatureClass1.Search(null,false);
    
             int lFld=0;
            
            lFld = pFeatureClass1.FindField(strFeatFld);
                if(lFld == -1)
                {
                    System.Windows.Forms.MessageBox.Show("field not found: " + strFeatFld);
                    return;
                }
    
            ESRI.ArcGIS.Geodatabase.IFeature pFeature = pFeatCursor.NextFeature();
            while (pFeature != null)
            {
                ESRI.ArcGIS.Geodatabase.ISpatialFilter pSpatialFilter = new ESRI.ArcGIS.Geodatabase.SpatialFilterClass();
                pSpatialFilter.Geometry = pFeature.ShapeCopy;
                pSpatialFilter.SpatialRel = lSpatialRel;
                pSpatialFilter.GeometryField = pFeatClass2.ShapeFieldName;
                pFeature.set_Value(lFld,pFeatClass2.FeatureCount(pSpatialFilter));
                System.Windows.Forms.MessageBox.Show("OID: " + pFeature.OID + ", count: " + pFeature.get_Value(lFld));
                pFeature.Store();
                pFeature = pFeatCursor.NextFeature();
            }
        }
        #endregion
    }
}
I hope this helps you....
Thanks and Regards,
Venkat Tammineni