Once a geoprocessing tool starts, it is not normally interactive. Interactive tasks are normally programmed with .NET or Python plugins, not with the geoprocessing environment.

In Model Builder, you could direct your user to do the selection before starting the tool, and use Get Count to make sure less than N features are selected before doing your processing. That could easily be done with Model Builder.

You could use a feature set to have the user draw a selection box before the tool runs and use Select By Location using that feature set, again followed by Get Count to check whether anything got selected.

This is one place where Python scripting is needed instead of Model Builder: using Python scripting you can use parameter validation so that the dialog will not let you click OK and give you a warning message unless you have less than a defined number of features selected in the input layer.