Pumps running in series will increase head (H), but not flow (Q). Pumps are operated in series when one pump acts as a booster to feed another pump. For example, pumping stations in a water network using two or more NB/NBE/NK/NKE centrifugal pumps in series will generate lift in the same manner as a multi-stage pump. When using the Grundfos sizing tool for the product in question, you can size identical pumps either in parallel or as single pumps in series.
If pumps are running in parallel, this will result in increased flow (Q), but not head (H).