So - as @alemabrahao has said; you need just one license, but the hardware must be identical:
MX75-HW x 2
country-specific power cable x 2
LIC-MX75-xxx-yyY x 1
Where xxx = license level, either ENT (Enterprise), SEC (Advanced Security) or SDW (SD-WAN+) Note all MXs in an Org must have the same license level.
And where yy = license term - either, 1, 3, 5, 7 or 10 years