ICMJE criteria on authorship stipulate
- Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
- Drafting the work or revising it critically for important intellectual content; AND
- Final approval of the version to be published; AND
- Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
The latter is conveyed by signing the JMIR License to Publish (see Where can I find the publication forms for authors and where do I submit them?).
If one author refuses to sign, he/she is likely not meeting all authorship criteria. See How to change authors after peer-review but before publication if authorship changes are required at this stage.
There is no maximum number of authors for a submission. However, if the number of authors is quite large (ie, 15+) please do consider that you may also list authors as "study collaborators", being part of a group (collaborative) author (What is a group author (collaborative author) and does it need an ORCID? and What are "collaborators" and where should they be listed?). Only authors have to sign the form, not individual collaborators (members of a group author).