In addition to the An-225, a few historic examples at five, a couple at six, and one (possibly) at seven.
I don't know how firmly worded regulations were at the time so I can't speak as to whether all of them were absolutely required to be in place for takeoff. As neither of the six-crew airliners entered service, the question might be moot for them anyway.
Two prototype airliners with six -
At least one operational airliner had a minimum of five -
- The Boeing 314 had a flight crew of five - two pilots, a navigator, a radio operator, and a mechanic. However, for the intended long-duration flights, it apparently carried two full crews plus two additional people - a chief engineer and a clerk. Relief crew are probably outside the scope of your question, but it's not clear to me if the two additional crew would have been "required" even for a shorter flight (the engineer maybe, the clerk probably not) - so the answer might be six.
And some military aircraft with five or more -
The Zeppelin-Staaken R.VI apparently had a flight crew of seven (commander, pilot, copilot, radio operator, fuel engineer, and two engine mechanics) - I say apparently because that list doesn't seem to mention gunners. There may have been some duplication of roles.
The Linke-Hofmann R.I (did not enter service) appears to have had a flight crew of six, but the details are sketchy.
I think the basic "flying" crew of the B-36 was five, after taking out everyone with a combat duty - two pilots, one engineers, radio operator & navigator; see this list - but it's hard to break out the exact roles and determine who was there for relief purposes. The XC-99 cargo variant also had a crew of five, which seems consistent with this.
The Me 323 transport had a crew of five not counting gunners (two pilots, two flight engineers, radio operator)