There doesn't appear to be anywhere, that I can tell, that indicates how much a Plank Pallet is able to hold. This is the case for viewing the description of the Pallet in the crafting menu, and when viewing the pallet after it's already been built. This information might be handy to have for players in case they want to carefully plan how many they will need/want.
https://imgur.com/VNnLsJh