I have a RB433ah AP with 2 x 5Ghz radios. I have a number of Ubiquiti Powerstation 5 clients. I have read various comments on the amount of clients the RB can support. My question is - is the limit on a per radio basis or on the RB itself. What would be the maximum clients for this setup.

In our experience we have seen that it is dependent on the wireless cards rather than
the routerboard itself, especially when you are doing NAT. More over you intend to use
ubiquity as CPE, thus creating a heterogenous network, instead of a homogenous, which
is better.

It depends on many factors including what type of client radios, polling mechanism, the speeds your selling etc. But in my experience, we are getting about 35-40 clients per radio card on an all mikrotik network. This is a mixture of 1Mbps and 2Mbps download speeds.

The rb433ah is a plenty powerful devise as an AP or router. Not sure if I would have it doing both though. You may be better off setting it as a bridge, and have a router for any routing or Nat.

Your real limitation will come down to the fact that none of your UBNT Gear is compatible with protocols where mikrotik can shine in PTMP. For example, Nstreme or Nv2. You are limited to 802.11, which will severely impact the number of clients you will be able to properly service.