@Jorge Marin Bus notation is supported for instances and for nets. Replace < > brackets with [ ]. See manual.

Thanks @Stefan Schippers, i just found a message on slack explaining this Question: Is mult = n and m[1:n] exactly the same un ngspice for a MOSFET?

I think they should be equivalent. The mult=n is more efficient as ngspice creates one device and multiplies currents, capacitances etc resulting from equations by n. In the second case n distinct devices are simulated, this should take more time. Try to verify this assumption with a small testbench.