Yousra,
Open SQL Server management Studio--> Connect to the instance-->Expand the concerned Database-->Expand tables--> Look for a table named Server_config--> R-click on that and 'Select Top 1000 Rows'--> Check the parameter "AUTH_KEY"
if the license string starts with:
1)sdepersonal......--> its a sde personal geodatabase (free with ArcGIS Desktop and has a connection limit of 3 users at a time)
2)sdeworkgroup....--> its a sde workgroup geodatabase
3)arcsdeserver.....--> its a sde enterprise geodatabase
My guess, you are using the SDE Personal and thus observing this limitation.