That's a tough one. Boas are ovoviviparous, which means they are contained in an egg within the mother until birth, but they still feed off of the egg yolk and have no umbilical attachment to the mother. Egg laying species (like balls) are oviparous, meaning they lay an egg. Mammals are viviparous, meaning they give live birth to a baby that was nourished from an umbilical connection to the mother. I looked at several online dictionaries and found this definition for litter in all of the ones i looked at "offspring at one birth of a multiparous mammal" (multiparous meaning giving birth to more than one offspring), so according to that only mammals can have litters. Since boa babies still technically hatch from an egg (even thought the egg is contained in the mother) they are still considered a clutch according to the information i found.

Either way, good luck with the babies.