Experts recommend that one visit one's dentist two times a year or whenever any unusual pain or bleeding occurs.
A. Experts recommend that one visit one's dentist
two times ---> there is no grammatical mistake, but twice is better and more formal than two times.
B. Experts recommend that one visit one's dentist twice ---> Correct
C. Experts recommend that
one visit
their dentist twice ----> pronoun-antecedent disagreement error. one(singular) doesn't agree with their(plural)
D. Experts recommend that
one visit
his dentist twice ---> B is better than D.
E. It is recommended by experts that one visit one's dentist two times ---> Incorrect. Unneccessarily wordy.