Hi!
When it comes to how many platforms you should post on, that depends entirely upon you.
Personally, I can't manage more than 3 platforms (Pinterest, Tapas, Webtoons). I haven't even really started yet, but thinking about managing them is tiring
. However, if you can manage more without getting stressed or tired by it, it's a good idea to do the maximum you can handle. That way you can figure out which platforms your story is performing best on. You never know which one will do well, and each platform has a different type of audience. 
If you can handle 2 but 3 would be tiring, then go for 2. If you can handle 5, then going for 5 platforms doesn't hurt!