The limitation per subnet is based on the unicast LID space. LIDs are 16 bits and unicast LIDs go from 1 up to 48K. Assuming LMC is 1, that’s 1 LID per device. To go above this limit, IB routers would be needed which would allow for multiple subnets. This extends the layer 2 (LID space) with layer 3 (GIDs) which means GRH needs to be included on any packet traversing multiple IB subnets.