Can I limit the number of images/files uploaded?

Yes! This feature is available with the 1.5.0 version of WP Job Manager — Field editor:

1.5.0 – September 8, 2016

• NEW limit number of files uploaded in file upload field
• This is handled both on frontend using JavaScript, and in backend via PHP to prevent anyone bypassing JavaScript.  Supports both AJAX and non-AJAX file upload field types for job, company, and resume file upload fields.  Support for packages will be added in an upcoming release.  Max files allowed shows next to max filesize (if field has no custom description)