I'm searching for an OpenWRT router that supports both AP + STA mode simulatneously. Additionally to that it should allow connecting around 50 clients in AP mode. Are there any routers you can recommend me? I am kind of overwhelmed by the possible chipsets out there. Thank you!
There are many that can do this in general, but he best option is to use multiple radios -- one dedicated to the sta mode, and the other(s) for AP mode operation.
If you want a specific recommendation, it would be helpful to know the context of your application... for example, is this a wireless range extender? a travel router? A normal routed configuration for a home/business with a wireless uplink? What is the uplink bandwidth? Are the client devices data-hungry (phones, computers, etc.) or are they IoT or other low bandwidth devices? Will there be a lot of traffic between devices and not much to the internet? or the other way around? or both inter-device and internet? What are the physical considerations (location/size/floorplan of space that needs coverage, power options/PoE, etc.)? What's the budget? Will there possibly be other APs added to this for additional coverage and/or client distribution? Do you need any wired connectivity, and if so, how many devices or will there be a switch? you can see there are a lot of considerations.
Sorry to be a bit of a party pooper, but this isn't going to work, at all.
With 802.11n and 2.4 GHz, you have a highly congested band of only three independent channels and relatively long range - so your neighbours' interference will hurt you. With 802.11n (and its accordingly relatively low base rates, used for beacons and similar), you end up in a situation where the management traffic alone (beacons, ARP requests, etc.) will get you close to 100% airtime usage around 32 connected clients (that ignores the interference from your neighbours, which you'd have to add!) and doesn't even consider your desired repeater setup(!), which doubles most of the traffic, nor the intra-client traffic passing through the repeater.
It's only 802.11ac/wave2 and 802.11ax devices, 'thanks' to the proliferation of many IoT devices in households, which has significantly added to the maximum number of connected clients, but even there you mostly gain in the 5/ 6 GHz bands (as you can run in greenfield mode there), profit from higher base rates, more bandwidth and channels, as well of lower range and interference from your neighbours.