Exact numbers are pretty hard to come by but pretty common.

The islamic slave trade took four forms, slaves from russia, poland and the asian steppes trhough crimea and the caucaus and into the middle east, slaves from the coast of africa north through the indian ocean into the red sea area and the middle east, slaves from west africa crossing the sahara to Libya and then into the middle east, and slaves caught by the barbary corsairs and then moved from the north african ports into the middle east.

So broadly speaking two african and two eurasian trading routes.

The problem with working out exact numbers is we only really have actual figures in the 18th and 18th century (where for instance we know 1,100 slaves arrived in tripoli by the land route in 1753.)

Mauney estimated the trans saharan slave trade was transporting 20,000 slaves a year in the 15th century, Wright reckoned 5,000 a year and Austen only 4,300. In the 19th century, Wright has 7,000, Austen 6,000 and Mauney 20,00 again.

The European trade from corsair raids is equally hard to pin down with numbers, Giles Milton had it at 14,000 a year at its peak. Robert Davis had it at 8,500 a year and I've seen reports that had it less than 4,000 a year.

The sea route from tanzania up the indian ocean, Susi O'Neill has at 15,000 a year. Bernard Lewis argues that it was very small during the heyday of the european trade and only started hitting 15,000 a year after the european trade routes had dried up.

Michael Khodarkhovsky had the russian slave trade through the crimea at about 4,000 a year. Alan Fisher had that at 7,000. David Ayalon has it as outnumbering the other three routes put together.

And all this ignores of course slavery of people already in muslims lands, which was arguably the biggest source. Like serfdom, there are estimated that nearly a third of some arabic countries were slaves.

But ok so depending on who you believe, the middle east, which was mostly arabic speaking but ran by turks at this point was, at it's worst, importing somewhere 15 and 35 thousand slaves a year from africa and between 7 and 21 thousand slaves from europe.

For comparison Thomas Kitchin has the atlantic slave trade from africa to the americas as involving 140 thousand slaves a year at it's peak. I've not seen any source suggesting the arabic slave trade was anywhere near as common as that.