How many streams does Tor attach to the same circuit, i.e. when Tor think that it is a time to build a new circuit?
I know that the lifetime of a circuit is 10 minutes (for new connections). But before that 10 minutes, can we attach all the coming streams to that circuit or there is a limitation on the number of streams?
I don't think there's any limit other than inherited from the protocol, i.e. 2^16.
The code does suggest that all stream IDs (other than 0) are acceptable for streams.
There is no limit to the number of streams in a circuit, but not every stream 'fits' every circuit. If a new stream needs to connect to an IP:port that your existing circuit(s) can't handle, because the exit node rejects it, you need to build a new circuit.
