No set number really. I'm T1 and I guess an avg is probably 15g of carbs. I wouldn't bother injecting for a snack of that amount. If my snack were say 30g, I'd treat it as a meal and bolus accordingly. I guess you're governed by your BG levels.
I have whatever I like so long as it is only 20g of carbohydrates,anything higher than that then I bolas but I usually have something like carrot sticks or cheese and crackers(2 cream crackers=10g of carbohydrates) or a small salad.