When creating:
However many you want it to have while still being balanced.
When advancing:
There's no limit that I know of on special abilities gained through using templates. If you somehow add 10 templates that each give a special ability to a creature that already has 5 special abilities, it has a total of 15 special abilities.
Well, when creating it, first think of what you want it to be like and what CR you need it to be.
There's no fixed number to the abilities a monster might possess, although I don't think I've seen a monster with more than 10 special abilities. And as far as it matters, I've seen in quite a level of detail all the monsters of Bestiary 1 and 2, since I've done error-seeking for those books (nothing official, mind you), and I'm about to finish the same for Bestiary 3.
On average (and without going to check) I'd say that, depending on CR, a monster has from 0 (CR 1/8) to 8 (CR 20) special abilities, half of which are usually universal monster rules (grab, gaze, trample, spell-like abilities* and so on), and the other half is peculiar to that monster.
But really, there's nothing fixed. Just make it as you see fit, and test it before letting it rampage the PCs.
* Here I considered multiple spell-like abilities as a "single" special ability, but if a monster has a wide number of them, you could consider them the equivalent of two or more special abilities.
When advancing, in most cases a creature doesn't get any new special ability, but maybe those it has increase in power in some way (some inherently, like save DCs, others by your hand, like a breath weapon's damage dice).
In other cases, they gain additional spell-like abilities, and in rare cases maybe they get an entirely new ability. Again, do it as you see fit and test it.
What sort of details are you looking for?
Ok how do you do that?No, that's exactly right. There are no rules or even guidelines for adding special abilities. You add as many as you like and you have to appraise the CR it will add yourself.Yeah I don't think that's right
Mostly practice and experience :)
This page may help: http://www.d20pfsrd.com/bestiary/rules-for-monsters/monster-creation