Well, I don’t know any way to really calculate it. It all depends on all the things you just said.
I have seen BizTalk process over 900 messages per second in a lab with a whole ton of servers pushing messages. We got about 100 message / sec on the receive side and 150 message / sec on the send.
I’d take a look at how you can scale BizTalk to meet your needs based on any actual results you might have now. You also might want to look at how you can adjust BizTalk settings (like water marks, hosts, etc) to perform best for your solution.
Hope this helps.