A regular 20lb grill tank will work for a 5 gallon batch. You should be able to do at least 3-4 batches on a tank.

I have this one for my crawfish boils, it's bigger than it looks in the pictures. It'll get 10 gallons to a rolling boil easily. https://www.amazon.com/dp/B091DBFLJ3?ref=ppx_yo2ov_dt_b_product_details&th=1