The limit you hit will likely be a practical one, not a technical one. Eventually the devices start "talking over" each other causing re-transmits and the throughput to drop faster than you'd like. At some point, the bandwidth and latency get to be too high for your application.
I'd guess it's in the dozens of devices somewhere for many applications. Probably a lot more if they aren't passing any data.
Is it really a "mesh"? That means that the devices all connect to each other, not having to go through some central point.
What are you trying to achieve? How much connectivity do you need? How much latency can you tolerate?
Edit:
Found
which suggests on the order of 40-50 per AP for 802.11n when the devices are using a single spatial stream. Another poster states "Rule of thumb is 25".