Can you limit the number of times a reviewer can get nominated?
The simple answer is no, but we did this for a good reason.
If you limited the amount of times a reviewer could get nominated to 3 times, that would mean that only the first three people who requested their feedback would have the opportunity to receive it. Even if they aren't the 3 people who would benefit most from it.
So, an employee might get nominated 8 times. Rather than limit this, you can inform reviewers to look at the list requesting their feedback and then choose the 3 or 4 (whatever number is best) reviewees they have the most meaningful feedback for and decline to provide feedback for the rest. Remember, feedback is not required. This declination can trigger an email to the reviewee so that they could request feedback from someone else. You can enable this in the 'More options' area of the 'Visibility and Anonymity' section.
Knowing that an employee was nominated 8 times is very interesting, and valuable, information for you and your team to have. Are they overextended with that many people wanting their feedback? Or, are they a rising star that you should fast track to a more senior position? Either way, it's a great opportunity to you to reach out to them and get a pulse for it.