Pumps running in parallel will increase flow (Q), but not head (H). Pumps are operated in parallel when two or more pumps are connected to a common discharge line and share the same suction conditions. Water pressure boosting applications use, for example, pumps from the CR range to ensure steady flow through a common discharge line. 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 series, this will result in increased head (H), but not flow (Q).