A "transport multiplier" in the free game C-evo is simply the number of air units that can be carried in one module on a Carrier or the number of ground units that can be carried in one module on an air unit or a naval unit.
Initially each is set at 2, from the discovery of Combined Arms for air transport, and from the discovery of Map Making for unit transport on naval units. Later advances make some increases.
In all cases, there is no numerical limit on the number of transport modules, but each module takes up part of the weight quota.