Bossman I don't know, I'm sorry. However, It's probably not related directly to the card.
I'm not sure if TVH limits the number of streams, but it could come down to the way the OS is configured and/or the throughput of your network.
Each card may contain 1 or more tuners and each of those tuners will be able to access 1 mux at a time. Each mux can have multiple services.
So if you have 1 card, you can only stream from 1 mux, but if your clients are all using services from the same mux, then your limiting factor could be the capacity of your network. I recommend experimenting to see what your network can tolerate.