The only answer is "it depends" and it could be 50, 100, 200 or more. It varies greatly based on client capabilities and what applications are in use, and what the RF conditions look like. No two deployments will ever be exactly the same. One of the critical figures of merit in wifi is "duty cycle" (channel utilization), and what drives the channel utilization is directly related to what apps the clients are using (data vs voice vs video vs mix, etc) and what are the capabilities of those clients (a/b/g/n/ac technology, channel width, supported data rates, etc) and also the RF conditions, because if there's a higher noise floor in the environment then clients will have a worse signal to noise ratio and have to use slower data rates. Lots of variables.
The real first step is always a site survey, but when it comes to estimating clients per AP, as a first step, figure out the types of client devices and the primary applications, and determine what the airtime consumption should be on a per-client basis, and from that you can do a back-of-the-napkin estimate on # of clients per AP. For example, just hypothetical, a 1x1 smartphone on a 20MHz channel might end up with a sustained max TCP throughput of 50Mbps, but a 2x2 tablet on a 40MHz channel might get 250Mbps. Just hypothetical, but the point is, that 1x1 smartphone could have 5X the amount of airtime consumption. Figure this out for the different types of devices and their capabilities and what application is in use (like 2Mbps video streaming or whatever) and for a mix of devices, your answer will be somewhere in between.
If you can estimate the amount of airtime consumption per device, then look at the bandwidth requirement and divide. Using the 2Mbps application requirement above, and say we're talking 50Mbps per device, 2/50 = 4% airtime consumption. Since we would consider an AP "maxed out" at, say, 80%, we divide 80 by 4 = 20 smartphones. But for those tablets at 250Mbps, we're talking 0.8% airtime consumption and 80/0.8 = 100 tablets on an AP.
Note, this is on a single radio, with a completely consistent mix of clients. Your mileage WILL vary. And there will always be a mix of clients, and your answer will lie in the middle ground. There's more to it, but there's a quick & dirty type of estimate.