Well, it all depends...

You need enough depth for the roots [I'd give them 12 inches' depth, though some might say less], and regular spacing, as though you'd planted them in the garden.

So if your containers are deep enough, you just plant the beans at 3-4 inches apart (whatever the recommended spacing is). You are limited only by the containers you want to use. For instance, I'm going to have seven buckets that are 11.5 inches in diameter and about the same depth. I will plant eight or nine cowpeas in each -- I have a few of them left over from last year. The new cowpea seeds will go into the garden. That makes 63 plants in those seven buckets if they all germinate and grow.