lol, no, i just took in the account that you might have other fishes in there. If you're just gonna have SD and a few polys, personally I would have about 8-9 of them and a few polys. well just my opinion. It also depends on what type of filteration too.
I wouldn't do more than six these guys also known as the disc tetra can get massive. I have seen them at the Detriot Zoo in an exhibit as large as 14 inch dinner plates.