Here's a reddit post which looks at some study data on this topic:

https://www.reddit.com/r/Masks4All/comments/1hnxfh3/debunking_the_myth_that_n95s_are_super_protective/

It's hard to give a specific number, since it'll depend heavily on the type of mask, the environment(s) it's used in, and what someone is willing to risk. It basically comes down to how contaminated the mask gets (degrading filtration performance) and how the fit deteriorates over time due to failure of various materials (stiffness of nose wire, rebound/attachment of foam seal, elasticity of straps). So when reusing, keep these factors in mind and monitor the condition of the mask carefully, replacing when necessary. Speaking from experience, it's pretty obvious when the straps on the Aura start to wear out and can no longer provide enough tension to get a good seal. If you really push the envelope, you can even end up snapping a strap, although from what I've seen you'd have to use them well past the point where the straps are obviously loose. I haven't tried it myself, but some people say the somewhat pricier 9210 (fabric straps) is better than the standard 9205 (rubber-like straps) in this regard.