There's usually no hard limit on any AP from any vendor regarding the number of connected clients. The same goes for MR45 and MR55. If you are planning for a high density environment there's a pretty good summary on the docs providing a lot of detail:
As stated by @hoempf and @Nydo89 the practical limit is much lower than the theoretical limit (128). There are a lot of factors in play here.
What's the rf spectrum like. A clean environment will perform better.
How is it configured: what's the minimum bitrate set to, larger or smaller cells
What kind of clients are being used. With Wi-Fi 6 the offered airtime resources can be finetuned to what the clients need. So there can logically be more clients if they have lower needs. But for that they also need to support Wi-Fi 6.
Also the presence of MIMO on the clients will infuence things. In any case, more antenna's is better of course as you can have more data streams.
What are the clients doing. Clients streaming all the time will take up more resources both in the air and in the processing power of the AP.
What's the uplink of the access point like, if not mGig you can easily hit a bottleneck there.
I´m not a huge fan of how much clients can a single ap handle types of discussions in general. However there is a pretty fresh Comparison Chart Meraki MR Indoor Portfolio out there with MR45 and MR55 included.