Seasons 1-5: There was no stated rule for Roadblocks. However, it is assumed that each team member had to do at least One, since Kim, Christie and Nicole of S5 all did exactly one. It is assumed this is the same for Season 8.
Seasons 6-7: The basics. Only 12 Roadblocks on the race, and no person can do more than six.
Seasons 9-17: No person can do more than seven, even though some races like 12 and 13 only had 11 Roadblocks in total.
Seasons 18-23: No person can do more than six Roadblocks before entering the last leg of the race. The Roadblock or Roadblocks of the final leg have no restrictions, except that if there's two one must be performed by each.
Seasons 24-onward: No person can do more than six Roadblocks before entering the 11th leg of the race. Any Roadblocks on the 11th and 12th legs have no restrictions, except that if there's two on one leg, one must be performed by each.
Fairly simple stuff, though I don't know why they got rid of the whole "completely even thing". Roadblock rules can get even more confusing in foreign versions of the race. The Israeli version keeps a firm 6-Roadblock limit, even though Season three only had SEVEN Roadblocks throughout the entire race. On the flipside, the fifth Latin American race has SEVENTEEN Roadblocks, with a grand total of Five legs having two Roadblocks on them.
Usually it's pretty fairly enforced (even with Connor, despite what the masses seem to think). Definitely the worst case was in the third Vietnamese season. In there, each team member can only do five Roadblocks before the final leg (The last Roadblock has no restrictions). However, the producers forgot how to count and one team member accidentally hit six Roadblocks on the antepenultimate leg. They didn't give him a penalty because he already received a penalty during the Roadblock so they were like "is ok..." This penalty was a grand total of 30 minutes.
Speaking of Luke though, I think me might have done okay. I'm not sure what you're talking about Kharpov. As Phil has said in his tweets, the Roadblocks of leg 11 and 12 have no restrictions. The only thing would be that if Luke performed the magic trick, then Margie would have to Skydive. (Would have loved to see that)