According to Greg Maxwell mining pool consolidation had reached the point of 22 large pools, as of the Bitcoin FDTN. conf. Nov. 2015.
What is the state of the current situation? Has there been a further worrisome decline in this metric?
https://blockchain.info/pools gives you all the info you need, we're not as bad as in the ghash.io days (>51%). I count 17 pools with >1% hash rate, however the four biggest pools have a combined total of >50%.
Another thing to bear in mind, is that people with substantial hashing power (such as Antpool), could just be hiding their actual hashrate in other pools, we've seen some pop up with some pretty large amounts of power in short order, and hundreds of petahashes swapping between pools.