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.
It is recorded in all four gospels that there were two, and only two, other men crucified with our Lord Jesus:
At that time two rebels were being crucified with Him, one on the right and one on the left. (Matthew 27:38)
And with him they crucified two robbers, one on his right and one on his left. (Mark 15:27)
Two others, who were criminals, were also led away to be executed with Jesus. (Luke 23:32)
So the soldiers came and broke the legs of the first, and of the other who had been crucified with him. (John 19:32)
I don't know how it could be any clearer.