I could be wrong, but I think the Astros went from late April to late August and for part of September with a 6 man rotation.
Verlander, Valdez, Garcia, Odorizzi, Javier & Urquidy. When they traded Odorizzi, McCullers returned from injury to take the 6th slot. I think Odorizzi missed a few starts in the middle, and they went with 5 for a stretch, but then later Hunter Brown got a couple starts that made a 7 man rotation for 2 cycles in September.
Looking at the GS numbers, Valdez had 31, which you can't get on a true 6 man rotation for a big chunk of the season, so maybe I'm wrong,
31 Valdez
28 Verlander
28 Urquidy
28 Garcia
25 Javier
12 Odorizzi
8 McCullers
2 Brown
Javier was in the pen for 5 games, but everyone else, except Brown were pretty much starters.
I think Ordorizzi & McCullers were the only guys missing time with injury. You'd expect 32-33 starts from Valdez, Verlander, Garcia and Urquidy had there been a strict 5 man all year.