Hi @lam,
Welcome to the :qiime2: forum!
Thank you for finding that! We've had several people go through the docs, but apparently it was misssed; I will update the tutorial. The correct flag is --kmer-map.
This depends on your database, region length, etc. There's no single correct answer and without having details about your specific primers, tables, etc, I don't know. What does your denoising look like? How long are those amplicons?
Finally, a business issue: please double check the forum code of conduct and don't double post or DM people. Public questions mean multiple people can find answers; cross posting can waste developer's time. We have a pretty good ecosystem for trying to make sure people get the help they need, but most mods are also full time researchers and are here on a volunteer basis in addition to our other work.
Best,
Justine