How many transactions can eCap process at an instance?

eCAP API does not impose a limit on the number of concurrent eCAP transactions, but the host application usually does. For example, Squid cannot have more concurrent HTTP connections than there are file descriptors available (but I am not aware of any "eCAP transactions per HTTP transaction" limit in Squid). Needless to say, all host applications will eventually slow down with more concurrent transactions in flight.

Please do not confuse blocking eCAP calls (at most one per host thread at any given time) with concurrent eCAP transactions (no artificial eCAP limit): A single call is blocking for the host thread but there could be thousands of concurrent eCAP transactions in existence at any given time, even with a single host thread. It often takes many eCAP calls to adapt a message, and the adaptation transaction continues to exist _between_ those calls (e.g., while waiting for more data from the client or server).