I'd like to know, too!
From the algae, it looks like your bowl has plenty of nutrients, so I'm sure 20 would be fine. The problem is, it won't stay at 20 for long.
About 9 months ago, I started with 10 shrimp in a 1 gallon bowl, similar to yours. Just for fun, I fed them one single speck of fish food, just once, but other than that, I haven't fed them. There are maybe 60 shrimp now (and that's after giving a dozen away), and they seem fine, but the population keeps growing. I still haven't fed them, and it doesn't seem like I need to so far.
Mustafa should sell all-female or all-male groups, so we don't have to worry about a shrimp population bomb.
