50-person crew, 90% men: can we use portable urinals to meet OSHA?

You still need the two toilet seats. Under OSHA 1926.51 the urinal is counted on top of the seat, not in place of it, so for 50 people the table works out to two seats and two urinals. Your math is right because 50 divided by 40 rounds up to 2.

The swap rule people remember, where urinals can stand in for some of the seats as long as seats don't drop below two thirds of the minimum, comes from the general industry standard (1910.141) and it only applies when no women use the facilities. You said almost all men, so even if you leaned on that rule it wouldn't fit your crew.

What we usually do on a crew your size is two standard units that each have the built in urinal, then add a standalone urinal or two if lines build up at break time. That covers the count and keeps the seats free for people who actually need them. Put the cluster on the gravel near the truck path so the service guy isn't dragging a hose across the site, and plan for more than the minimum once framing crews and subs stack up on the busy weeks.