The Clerk shall accept for filing only nominating petitions on official blanks issued by him containing the required number of signatures for candidates having those qualifications required for elective city officers by this Charter. Within five days after the filing of a petition, the Clerk shall determine the sufficiency of the number of genuine signatures on the petition. If he finds that any petition does not contain the required number of genuine signatures, he shall immediately notify the candidate, in writing, of the insufficiency of his petition. If the Clerk does not so notify a candidate, his petition shall be deemed to be sufficient as to the number of signatures thereon and not subject to challenge with regard thereto. No additional signatures on any petition shall be received by the Clerk after the final date and time fixed for filing nominating petitions.