They are small, but swim pretty fast and in a 64 litre I'd go for 2 max to be sure you have constant supply of algae.
When you are picking them, make sure they are nice brown colour on their backs and not black and white.
I picked 3 a few weeks ago and the two with the black and white look died, the third one was perfectly fine. I went back to the shop and asked whether the brown and black/white ones were different species(yes, the colouring was like the more expensive zebra ottos, but of course not that pattern) and apparently the black and white ones were the non-settled new stock they just had put in. I picked another 2 that had brown backs instead and the difference in activity and behavour is great. They are doing perfect, trailing poos longer than their bodies but still not eating veg.