We know for certain that Jesus and two other men who were convicted of crimes were crucified on the same hill. Is there any basis for believing that there may have been other convicts?
Not within the Biblical accounts, which is the only recorded history of that particular day/location.
There's no reason to dogmatically state that there were only two others with Christ. It's certainly possible that others were there, but they aren't recorded as being there. Other than pure speculation, there's no reason to believe there were others there, and there's no reason to say that there weren't others there. The Scriptural accounts may only record these two because they were the only two right next to Him, and the only two that the writers bothered to mention. Or it could be that there were only two.