An exception of type 'System.InvalidOperationException' occurred in System.Data.dll but was not handled in user code

Additional information: Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.

<add name="cs" connectionString="Data Source=.;Initial Catalog=pms; User ID=sa; Password=sa@123"  Max Pool Size=20;/>

i tried it, but showing error in web config