That depends. On a small tank with All 4 Reef dosing or kalk, you'd only need one doser. For two part dosing (alk and cal), you'd need 2 heads. If you're going for 3-part, go get the four dosing and the extra one is for trace. Lots of potential combination here and it really depends on what your tank needs. I did have 7 heads at one point but it's not easily managed and prone to issues so I would go for an app driven dosing head instead of the cheaper jaebo ones. Most folks on moonshiners manually dose anyway, so just focus on either two part or three part.