The min number of distinct sequence numbers required to ensure correct operation of the ARQ scheme is
Your interpretation of bidirectional is correct.

First of all, it should be bi-directional. Here is the reason. The receiver window size M is not equal to sender window size N so it must be bidirectional. If it is unidirectional, then the receive window should be 1 for Go-Back-N protocol and N for Selective-Repeat protocol.

Then it is very straightforward: the minimum sequence number should be N+M. For example, the sender could choose [0, N-1] as sequence numbers while the receiver could choose [10000, 10000+M-1] as sequence numbers.

One thing to note, this problem is not strict. If you use Selective-Repeat protocol, then the minimum sequence number should be 2*(N+M) to handle both new packet and a retransmission.