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A lot of people say groups of 10 for all Corydoras species, but some say that it's OK to keep 1 or 2. How many is the ideal number? (The cory I am keeping is C. similis.)

I'd go for about 6 of them. More is better. 1 or 2 is too few for corys to feel happy and comfortable (they like to have "friends" around).

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Mats

There are 2 important factors for corys.

1 - they are best in groups. 1 or 2 really isn't good enough, 3 or 4 is better, 5 or 6 is IMHO the minimum group size and if there are more then all the better.

2- they need the right substrate. A thin layer of round grain sand is what is required. They can do ok in bare bottom tanks or over gravel but this is far from ideal as it is unnatural for them.

A big shoal of a single species of cory really is an impressive sight (the pic by Larry [Apistomaster] of his hastatus springs to mind!!!)

Here is a view of one of my C. hastatus permanent breeding colony set ups. You can see that even very small specimens join the main school once they are past their larval stage. During that period, the larvae hide among the needles of Hornwort and inside the hollow ceramic media I supplies for the larvae to find refuge until they complete their metamorphosis into miniatures of the adults which occurs some time between when they are about 7 and 10 mm TL.


Young Cordoras sterbai will shoal well when they are frightened or just recently moved to a new tank as shown below.I put about 100 juveniles about 3 cm TL in a 75 gal with Heckel Discus. Most of the Corysdoras show strong schooling behavior when kept in large groups and even that behavior tends to be seen less and less as they grow older but keeping 10 or more of a Corydoras species is always a good idea.

Now I only use L134 and L333 plecos as my primary bottom feeding fish in my Discus display but at the time I took the photo of the C. sterbai I had around 1000 I was selling. I no longer raise any Corydoras except enough C. hastatus to keep the species going as they can be difficult to find for sale.
I find a Leopard frog L134, to cite one example, make really good bottom feeders in Discus display tanks. One of these small species adult plecos can eat as much food as 6 Corydoras and need the same warm water temperatures as Discus the warm water Discus require so they have replaced Corys in my Discus tanks. Not all Corys prosper in water temps above 82*F/28*C. Many of those Corydoras from Rio Negro tributaries are well suited for warm water. C. sterbai is the only one which is mass produced and many of the attractive Corydoras species are from the Rio Negro system are among the most attractive in the genus, imo. Except for TR C. sterbai, these other species are mostly wild caught imports which cost more than C. sterbai. I am fond of Corydoras sp C-121, C. duplicareous and C. adolphi. All of which have bred for me but I haven't been very successful. I do not have wet thumb when it comes to Corydoras species that comes close to Frank Falcone's. He knows much more about Corydoras keeping and breeding than I ever will.