Hi Prasad,
this is a typical problem with portals which intensely communicate with AS ABAP.
The number of connections between two systems is limited on caller and on receiver side. Typical default value is 100.
To increase the number on caller side (portal) there is OS environment variable CPIC_MAX_CONV, see http://service.sap.com/sap/support/notes/314530.
To increase on receiver (ABAP) side there is profile parameter rdisp/max_gateways.
We typically set both values to 1000 in our systems without experiencing any performance issues.
Have fun,
Lutz