Carrier Aggregation is a technology to combine the multiple Basestations(NodeB, eNB, gNB) with different carrier frequencies to increase the data throughput. This became a widespread technology in LTE Advanced, but even in 3G is similar technology called HSPA DC(Dual Carrier) and will become a common technology in 5G from day 1.
How many carriers can be aggregated ? In 3G, as the term DC(Dual Carrier) implies the number of carrier is 2. In LTE, 3GPP specification defines the max 32 aggregated carrier in concept, but as of end of 2018 I see the max 8 aggregated carriers of real implementation. In 5G, max 16 aggretated carriers are defined from day 1, but it will take a couple of years to see the real implementation of 16 carrier aggregation in 5G.