I believe it was around 300. If I read correctly, he wanted to free his slaves but his wife did not want to free hers. He did not want to break up the families that had intermarried by freeing some of the slaves but not all.
Slavery was flat out wrong. But, the constitution did not make slavery legal. So, they did not get it totally wrong IN THE CONSTITUTION, nor did they get it right by strictly forbidding it intially. Some of the founding fathers were for and some against. Unfortunately, it took another 170 years for everyone to get it right in our code of laws to ban all discrimination based on race and sex (actually there are still too many people, both white and black, that can't look past skin color in dealing with fellow human beings).