I'm sorry to say that I don't think this is knowable. We have decent demographic data on enslaved populations (because it mattered for apportioning the House of Representatives) but slaves who tried to steal themselves is a different story. The 1860 census has a fairly low number, around 400, but that's enslaved people at large on census day who were reported as such by their enslavers.

A better proxy would be fugitive slave ads, but enslavers usually didn't post those for a few weeks because of a larger issue: many of the enslaved who absented themselves from their enslavers' property probably didn't plan a full escape from slavery. Rather they wanted something like a respite from abuse or a chance to visit friends and loved ones who didn't live on the premises. Slaves could make this a semi-regular thing, vanishing for a few days or a week and then coming back in with an apologetic attitude. Some enslavers would shrug it off or assign only token punishment, but it really depends on the people involved. Some were just brutal from the start.

Escape may sound preferable to us, because slavery, but for most enslaved people, most of the time before a British or Union army shows up, it's just not a realistic option. An escape means permanently leaving whatever friends and family you have, which is one reason the enslaved people we know tried it tended to be young and unattached men, and whatever passes for a home. The repeated destruction of enslaved families and communities is one of the thing the enslaved told us they feared and found most painful. At least if you were beaten, you could still have loved ones to give you emotional support.

Then one needs to navigate the hostile South, where the law empowers any white man to challenge you and demand proof you're abroad with leave. If you can't answer to his satisfaction, he has license to apprehend you using about any force he cares to try. Then one has to dodge the slave patrols, which are intermittent but literally have catching people like a fugitive slave as their main job. Somehow one needs to find one's way to a free jurisdiction, which was quite a ways for most to go on foot and posed a greater risk of discovery if one used a riverboat or railroad. If one was very lucky, one might know a free black or (more rarely) a white person who would give aid in all that. Except for the willingness to look the other way and offer quiet help on the part of other slaves, which could be a considerable advantage but probably isn't going to get you hundreds of miles on its own, most slaves hoping for a proper escape are on their own at least until they hit a big city on a busy trade route. They might be stuck until they reach the North or Mexico.

That's a long, dangerous trip with an uncertain end. Best case, one lands where there's a large free black and white antislavery community that will help with a new start. But you can never go home again and opportunities in the free states are often limited by fairly vicious racism. Canada might seem a better chance, but that's still farther to go. Anywhere short of there leaves a person always at risk of recapture or simple kidnapping.

We can say that many slaves fled despite these obstacles; some of them did reach the North and Canada, but an exact figure could only be a guess. The people concerned had good reason not to keep precise records, especially after 1850 when aiding fugitives became a federal crime. The estimates I've seen usually run around a thousand a year escaping to the North. That may sound like a lot of people, but out of a slave population of four million it's a drop in the water. Southerners in the border states screamed bloody murder about how they were losing so many people, but until armies friendly to fugitive slaves entered the region (in the Revolution, the War of 1812, and the Civil War) the losses were not enough to really weaken the system. The natural reproduction of enslaved people plus smuggled imports (which largely dropped off by ~1820) more than made up for the ones who stole themselves to freedom.

Sources

The Slaveholding Republic by Don Fehrenbacher

Gateway to Freedom by Eric Foner

River of Dark Dreams by Walter Johnson

The Peculiar Institution by Kenneth Stampp

The Internal Enemy by Alan Taylor