The records show 5 ~ but there are 2 others out of that first 15 who didn't own any slaves while they were in office.
George Washington got Virginia's statutes revised so an owner ~ himself for example ~ could free his slaves in his will, which he did.
A large proportion of the early settlers of Ohio, Indiana, and Illinois were from Southern states. Some of Virginia's Revolutionary War veterans got land in Ohio. So it would not be surprising if some of them brought their pro-slavery attitudes with them.
I read once that after Illinois became a state (and therefore was no longer bound by the Northwest Ordinance's prohibition of slavery) that there was a vote on whether to allow slavery and it lost by something like a 4-3 ratio. Surprisingly close since slavery had never existed there (except among Indians and perhaps French colonists). Of course Illinois people later hired slaves from Kentucky so some slaves worked in Illinois even if it was a free state.