When considering pools as Whisky said, the first question is opposed or threshold?
How Critical is this roll to my team?
How good do I want this character to be?
What level of competition do I expect to see?
What's the table expecting?
Do I endanger the team by having a pool this high? (If your 3 times as hard to hit or damage as everyone else, your going to get your table in trouble, try not swing so big your GM needs to being artillery fire onto the group to threaten you.)
Basic thresholds:
@Below 6 dice, I'm including this skill just b/c it says something about my character it doesn't matter if I am successful when I roll.
@ 6-9 dice for something that won't be opposed but it is important enough that it will get rolled, and failing is could have some level of consequences (unopposed drive rolls, basic etiquette for non-faces, the ability to find something very basic on the matrix)
@12-16 dice professional level- for threshold where you're pushing up values for some reason, in opposed you have a chance against competent Oppo. This a meaningful core skill, something your character has done ever day of his or her professional life. (Most serious competitive rolls)
@18+ Only need for Opposition rolls, for instances when you're pushing envelop, begin to keep an eye on limit at this stage. Even in opposition rolls you're pushing up values. For most skills this will be the province of focused builds, and are only needed on Archetype core build components, (Negotiate rolls for faces, Shooting tests for Sams, Melee rolls for Adepts, hacker focus area, etc)
Exceptions- Somethings are harder to raise and some are easier. Armor is a great example of something that's fairly easy to push way up, if you're in a game where combat is reasonably likely i'd always recommend going for soak pool in the 20ish range. It's not always going to be enough but it will give ya fighting chance. Magic is an example of something that is expensive to raise, pushing past the 14 dice range starts getting pricey I wouldn't advice trying push that roll too much higher at creation.