* olizilla Thanks for your answer. 
first, I don't understand why I reached 27000 connections in a first place with the default connection manager configuration [600-900] and 20 sec gracePeriod ?
secondly, I reduced the connections to [200-300] and it went up to 3000 connections during a few minutes  and now it's around 200... 
Any idea what's going on.
By the way go-ipfs version 0.4.20