Is there a maximum number of skill points in Dragon Quest IX?

There is no max cap as you can reincarnate a character to go back to level one. You will keep all skills and can earn more points. In fact you need to reincarnate in order to learn all the skills as you don't earn enough just by reaching 99.

A character will gain a total of 200 skill points by reaching level 99 in any particular vocation. There are twelve vocations, so you can reach a total of 2400 skill points by raising all vocations to 99. There are a total of 26 pools to put skill points into: 12 each specific to a single vocation, 13 for weapons, and one for shields. Each skill pool requires 100 skill points to be brought to the very top, so it would seem that the 2400 skill points wouldn't be enough for the 2600 skill points required to max all skill pools, however...

Revocation lets you bring a character's level back down from 99 to 1. This does not discard any skill points (allocated or saved); it only resets stats specific to having reached level 99. Furthermore, it lets you collect 200 more skill points by bringing the vocation to level 99 again. So revocating a single vocation just once will be exactly enough to fill all of your character's skill pools: 2400 + 200 is 2600.

You can also revocate any vocation more than once, to a limit of ten times. Thus the total maximum number of skill points that can be gotten is (12 vocations) * (200 skill points per to-level-99) * (1 initial to-level-99 + 10 revocations); this simplifies to 2200 potential skill points from each vocation, for a total of 26400 skill points.

However, the maximum number of saved skill points is, according to several GameFAQs posts (e.g. this one), 2600 saved skill points. Uncoincidentally, this is exactly the number of skill points needed to fill all skill pools. You could collect all 26400 skill points and get yourself locked out of ever gaining any more (since you couldn't revocate any of your vocations again), but you would still have enough to raise all skill pools to the max, even if you'd never before allocated a single skill point. That said, you'd then have zero saved skill points, and be left with no way to gain any more skill points. I suppose you could call this a "perfectly allocated" character, but achieving it would mean revocating 120 times, which would be a pretty long task!

Disclaimer: I have not totally double-checked this math, but I think it's all right (I have for sure checked the number of weapons and vocations).