You cannot participate in any game and maintain RS status. A "redshirt" is actually granted at the end of the year, not the beginning. If a player has a debilitating injury or illness in the first half of the year, did not participate in more than 2 contests or 20% of the season, they are eligible to apply for a hardship waver to gain an extra year.
This. It's more 20% than the number of games. So, they can be generous in granting the waiver, see Golden who received his waiver though injured during the third game.