You can put 1 at a time, and that really is all that you should ever put into a QT no matter the size. The idea of a QT is to give the new fish time alone to acclimate to capture with minimum stress. Additionally, if one fish gets a disease, no more than 1 fish need be exposed to it, or treated.
The only exception to this would be paired fish, or schooling fish. Obviously, a 10 gal tank really limits ever putting schooling fish in there.
Good job, deciding to set up a QT!