What are the number of Muslims involved in the crusades?
The Crusades occurred over a period of about two centuries and there is some debate about which events deserve to be included as part of the Crusades. Due to these factors and the general imprecision of historical records in the Midlde Ages, the precise number of people involved on each side of the various conflicts is disputed.
That said, we do have some approximate numbers; it is believed that somewhere between 1 million and 9 million people died on all sides during the total of all the Crusades. Deaths appear to have been about evenly split between the Christian and Muslim factions, and army sizes also appear to have been comparable. Casualty rates in the Crusades were extraordinarily high (probably due to the religious fanaticism and quasi-genocidal intent motivating them), so as many as 50% to 75% of soldiers deployed died. This means that the total number of soldiers on both sides was somewhere between 1.25 million and 18 million. Thus, the number of Muslim soldiers in particular was probably somewhere between 500,000 and 9 million.
World population at that time was about 300 million, of which probably 50 million in the Middle East; but all soldiers were men (only half of people are), and there were also 10 generations during the Crusades; so somewhere between 0.2% and 4% of all Muslim men alive during the Crusades were soldiers involved in the Crusades. This is comparable to the proportion of men who are part of modern military forces (about 2% of men in the US are in the military, along with a much smaller proportion of women).