The number of applicants this year increased compared to last year.
応募者 is used for applicants responding to a recruitment or contest, often with a focus on the act of applying. 志願者 carries a nuance of personal aspiration or volunteering.
Compound of 志願 (shigan, 'application' or 'volunteering') and 者 (sha, 'person'). The word is transparently derived from its components.