In the end of the day everything comes down to safety and security of your funds! This is the best advice here, don't put all the money in one exchange! There're many good exchanges, but all of them can be hacked (nice article by the way). So spread your money on different platforms, try different platforms, it's a good thing, you will have the opportunity to check the platform, all its pros and cons. I find that to be the most healthy approach, to do everything by youself and to try to learn as much as you can in that process.
Its not that necessary for you to put up on various platforms 1-3 would be enough and beyond that would already be exaggerated.Its always been suggested and recommended after all the years
that never ever put your funds on an exchange for too long no matter how good and reputable it is, you cant still be sure on when it would be hacked or would become scam and other sorts.
For now there are known exchangers and even them arent excluded on the chance on getting hacked but luckyily they do able to compensate or patched up on whats been lost.
I agree with you that 1-3 is optimal, I am active on 2 exchanges! By that active I mean I use them daily or at least once in 2-3 days! But in the same time I am registered on many others, and I used them at least once, or tried them for some time. For over 3 exchanges you need to be more involved, it will take more time and energy. But people do that, competitions are popular and people like to search for good competitions to participate, that requires registration on different exchanges. For some time I had fun with that, but I am more into wagering competitions in gambling than in trading!