So I have this line of sentence:
This only applies to the first 100 people signing up.
When I see the word "applies", I am confused, is "first 100" plural, or singular? Or it should've been "apply"?
The subject is this, which is singular, so the verb is singular (applies).
The thing which is plural "the first 100 people signing up" is not the subject or even the direct object, but the indirect object of applies. In English, verbs agree only with their subject, not with any other argument.