FCLoopBasisGetSize[n1, n2] returns the number of linearly independent propagators for a topology that contains n1 loop momenta and n2 external momenta.
FCLoopBasisGetSize[1, 0]1
FCLoopBasisGetSize[2, 1]5
FCLoopBasisGetSize[3, 2]12
FCLoopBasisGetSize[4, 1]14
The third argument (if given) is simply added to the final result.
FCLoopBasisGetSize[4, 1, 1]15