In this question, someone is asking how to tag more than 50 people in a picture. He even has a link to someone who has done it.
I can only tag up to 20 people.
How are some people able to tag more than 20 people?
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You can status tag up 10 users.
You can location tag up to 20 users.
You can photo tag up to 50 users.
You are talking about different things.
I assume you are talking about the location tag.
As always with actions that only one user is able to accomplish but the masses cannot; it's best not to try to attempt it. The correct answer is: it is not possible to tag pass the limit. And if it is possible you put yourself in the "might get blocked/banned from Facebook for crossing some abstract clause in the Facebook TOS" boat.
As for the reasoning for the limits comes down to distribution. You are using a hammer to fry your eggs. In most cases, these actions come from users trying to promote an event, spammers or an irrelevant photo of a cat/cartoon/joke.