Q: How exactly do I fill the right number of keywords in the form for submission to the journal?

We responded to a very similar query recently. You may go through it to see if it answers your question: How do I solve the problem of providing keywords during my submission to Materials Letters?

If not, you may of course write to the journal for support.

For help with other aspects of submission, you may also to wish to know about R Pubsure, a sister brand that a performs a check on your manuscript to ensure it’s ready for submission, and even more importantly, to help minimize or even avoid desk rejection. The tool runs a series of checks (such as for language, authorship information, and references) and provides a report that you can refer to for improving your manuscript until it’s absolutely submission-ready. You may know more about R Pubsure here and view a sample submission-readiness report here.

Hope that helps. All the best for your submission!