We have generally found that anything over 60 devices connected to an access point (30 per radio) causes noticeable quality issues and it can get bad before that. If the devices need to use VoIP then the quality degrades much more quickly. I have seen ~100 devices remain connected on an MR42 or MR52 but we get a lot of complaints when that happens.
I'd start off with 10 APs and expect issues if the user's aren't evenly spread about or the environment has low ceilings, pillars, metal sheeting etc.
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