The "safe" limit for asbestos in materials is 1% but how many fibers is that? Recently, I am afraid I have had exposure to asbestos when a block of ceiling from my apartment room fell off. It was a pretty old building that had not been remodeled or anything since it was built before they knew about asbestos.
The 1% refers to a weight comparison. So in one pound of material, there would be 0.01 pounds of asbestos. In 1000 pounds of material, there would be 10 pounds of asbestos. There is no way to calculate fibers or more importantly fibers released since that depends on the type of asbestos and the building material in which it is contained. Now, the ONLY way to know if asbestos is even present is to have the material tested. Age of the building is only a clue and a really, really old building is less likely to have it. The key dates are 1940-1980 where asbestos was used heavily. The fact that you indicate it fell in a block, makes the likelyhood less and limits your exposure. Lastly, asbestos in a natural mineral so you always have a background exposure. It is the amount that gets in the air that is important. What you describe, even if asbestos would be a minimal exposure and would not add to the everyday exposure from the ceiling. You need to have the material checked by a lab.