Hi,
See this VPI Gateway Considerations post, it explains it very nicely.
The Gateway has one TCA, this is an internal block within the switch.
The gateway can switch IB to Eth traffic (from any number of nodes), same as regular Ethernet or IB.
The impact of fact that there is only one TCA is related to routing on the infiniBand side and the utilization of the links connected to the gateway (depends on your topology)
See for example consideration 2 in VPI Gateway Considerations.
let me know if you have more questions,
Could you share your network/setup so we can discuss more into specific details?
Thanks,
Ophir.