A method and system for controlling how many carriers to aggregate together for carrier-aggregation service of a UE. A base station could take into account how many MIMO layers a UE would support respectively for each of various candidate quantities of carriers, and the base station could select the candidate quantity of carriers that would result in use of a highest aggregate total number of MIMO layers. Further, the selection could involve mapping UE-reported rank to a quantity of carriers, with the rank being inversely proportional to the selected quantity of carriers. The base station could then configure carrier-aggregation service of the UE with the selected quantity of carriers.