How many if any games can I have someone I promised to start come off the bench. I am honestly only wanting to do it once.

The general consensus is you won't suffer a WE drop if they start 20 or 21 games.  That said, the more starts they miss early in the season the more likely you'll see an early WE drop (which can be recouped with consistently starting him after that).  If you're only going to have him come off the bench once, you'll be fine.

I also found that if you don't start him in 2 consecutive games, no matter how late in the season, that will set off a negative reaction.

Also keep in mind that Conference Tournament games CAN matter in recovering a damaged WE. I had a guy drop around 35 WE points because I did not start him for a 4th time (this was around game #23). I started him the rest of the regular season, but his WE did not recover. I started him the 1st Conference Tourney game, but no WE recovery. Fortunately, I won that game and got to play a 2nd Conference Tourney game. I started him that game, and even though I lost, his WE recovered 90%.
I don't think it is required to start a guy with Promised Start in the Conference Tourney, but if you have had a negative reaction and are close to the threshold, it may end up making a difference.

While they CAN matter to recover lost WE, it is NOT necessary to start a guy during the conference tournament or other postseason play in order to fulfill your promise to do so - it only counts for the regular season.

the answer here is not 100% known, but roughly you have to stay above 80% starts. 20/25 is 80% and i think that last check for pinging you on promises is just before the season ends, which lets people squeak by with 20/26. also you don't have to start after the regular season but apparently there is one last check to un-ping you on promises sometime during or after the CT. but anyway, stay above 80% all season and you should be fine. 

during the season, if you say, skip starting him the first 6 games, his work ethic absolutely will drop. you will, however, be able to undo some of this damage, recovering the lost work ethic. but you lost work ethic growth and individual growth during the time his work ethic is lower. the guy won't quit, but it still hurts. the comment about not starting 2 games in a row always making them complain is definitely not true, ive done it a ton of times, early and late in the season.