On the new roadmap, they plan to release a way to "downgrade" your workers/soldiers into carriers. But I do like the idea of setting a "minimum" number so that you never run out.

For now, just make sure you are building a basic econ up before you expand too far. As long as you have a few houses and a tavern setup with the basic food resources you should always have some growth and will never run out of carriers.

I play on random maps and sometimes its extremely difficult to get a basic setup going but if you just get pumpkins and water that will jump start your growth at the start of the game. Once you start to expand you should be able to find a meat like deer or rabbit that will work for your carriers. Trout also works.