I don't think you've got room for any more fish if your tank is 10 gallons. However 3 is a good number for corys in any tank less that 20 gallons.
Wat about the size of the tank? Paulf u cant just have 5 cories in say a 20gallon tank just for the fun of it. Id say if there are at least 3 in a small space they should be ok. Just my opinion