Hard to say, you can gauge the amount of CPU cores through the difficulty chart and the number of workers or addresses at the varying pools, but that's not a definite indicator. My gut feeling would be 20 to 100, with a dozen major hashers tops.

The difference is the slower decrease, meaning you can still mine FRQ effectively for a while, while QRK mining got rather pointless a couple weeks after launch and is completely pointless now unless you're a QRK holder and want the network to live: rewards of 2 QRK, and negligible fees.

QRK was mined way too fast, and the network now lives only on subsidies from the big holders, it isn't clear if FRQ is slow enough (but it's not marketed/advertized enough, that is clear).

So yes, FRQ is quite concentrated, but you can still get in, if you want to (and at the moment it's cheap on poloniex, so I wouldn't bother mining too hard)