Keep in mind that inventory slots are abstracted. You can also rationalize that Magicians and Clerics are going to carry around smaller things because they need to be able to work freely with their magic or be unburdened by unnecessary baggage in service to their faith. At any rate, according to the Gear list in the book, each use of supplies takes up one inventory slot. However, also keep in mind that players can always use their scavenging or hunting skills to gather these same supplies and refill those inventory slots once emptied, allowing for more use of skills, more roleplaying, and more chances for adventure. Feel free to create a magic item or special power for your wizard or cleric that allows them to stack up to 3 or whatever supplies into their belt pouch if you’d like. Hope that helps.