Players 13, 14, 15...

Are these players limited in some serious way (except they cannot be played by real player)? Can I use them as creep fractions (5 total, player 12 is unused, so I would have to use players 12,13,14,15 and 16 ) ?

Depends entirely upon whether you speak in GUI or Jass.
In GUI, the players you refer to are 13, 14, 15, and 16
In jass, the players you refer to are 12, 13, 14, and 15.
The only reason they are different is because GUI uses a Converted player number so that it's easier to understand.

I'll discuss this in terms of 12, 13, 14, and 15 for clarification.
Players 12 and 15 are generally unusable, they have their own hardwired AI attached to them.
12 is Neutral Hostile and 15 is Neutral Passive, which should be rather self explanatory.

The other two, however, can be put to a very good use.
In many AoS maps these days, you can see the game is 6v6, and yet the computers still exist.
This is because Neutral Victim and Neutral Extra are being used.
(Players 13 and 14 respectively)

It's quite easy to set up if you're interested, also depending on what kind of map you're working with.

Good explanation, thanks a lot.

+Rep