It depends. But here is what I've seen.
Assuming your SEPM is installed on Windows Server, the number of pushes to clients will be unlimited because server does not have a limit on the number of connections that can be opened. So if you select 100 clients, it will push 100 at the same time.
If you installed your SEPM on XP, the limit of concurrent connections is 10, so if you want to push to 100 clients, it can only do 10 at a time.
The amount of clients you want to push to will depend on how big your pipe is. If you have a slower line, I would suggest limiting the amount of clients you push to at the same time.