There's no hard limit -- it's solely based on moderator's discretion. We tend to protect very popular questions that are somewhat "soft", that is, many people might have similar (but not exactly the same) observations and opinions, which don't really contribute much additional information that hasn't been mentioned in another post. Popular questions attract a lot of new users, which might be under the impression that SE sites are similar to traditional forums, where 20 pages of posts is not unusual. SE network is different, so in the interest of preserving high signal-to-noise ratio, some form of throttling new answers is desirable -- hence the protection mechanism.
Concerning where the threshold is set: for me personally, more than 5 or 6 answers to a question (even if all are good) is typically a warning sign that protection should be considered. This is not a hard limit by any means, just some practical limit that might indicate a question is in trouble. Protecting is also much more likely if there are deleted or converted to comment posts from new users.
The reputation threshold to post is so low, that for any user who has already contributed to the site should have enough to answer a protected question. By all means, if you have a genuinely new observation, opinion or a reference different from the other posts, feel free to answer a protected question; if it's mostly the same, it might be better to add a comment to a relevant post.
