Answer:
There is no error in subject-verb agreement
Step-by-step explanation:
Some people think - some people is plural, verb is plural - ok
Vegetarians don't get - vegetarians is plural, verb is plural - ok
a number of plant-based foods contain protein - a number of (...) foods is plural (even though "a number" is singular, but when we're talking about a number of people or foods or anything, the subject is, in fact, plural) and the verb is plural as well.