May? You gotta be kidding.
The MINIMUM trips for a DIY plumbing job is 3: one to buy the parts you think you need, the next one to buy the parts it turns out you really need, the next one to replace the parts you broke or dropped somewhere you can't get them out of.
There IS NO maximum number of trips.
PS. The pros are laughing, but they shouldn't. They do the same thing, except they've got more parts on their truck and don't have to go to the store.