I went 8 for a few reasons

• In a 16 team conference, 8 is a nice way to split them
• it still leaves Hawaii with 4 other games to go play on the mainland (they could either be paid for body bag games or play more evenly-matched opponents)
• it means pretty much nobody besides Big 12 teams is going to Hawaii, more or less locking other conferences out of games there

It would certainly be possible to scale that number though.  4 Big 12 teams per year would still allow 4 Big 12 games in Week 0, getting most of the benefit, and would cut it down to one visit every 4 years for each team.

6, as you suggested, would probably be a nice medium, but I mostly just didn't want to think about scheduling since 16 doesn't evenly split that way.  But it would still allow the Big 12 to become the major players in Hawaii and would have most of the same benefits while perhaps being less of a commitment.

6 teams would mean that 48 games over 8 years, so that would have teams playing in Hawaii three times every 8 years, which would honestly probably be better than my suggestion