First thing to make note of, the chatters endpoint returns a list of users connected to chat and in that streams channel. this means that it is NOT a list of viewers. There will be some viewers who are not logged in to Twitch or who may be watching on an embed where there isn’t a chat which means they wont show in the list, and the list will also include the many bots that join all channels on Twitch which are not actually viewing the stream at all.
Secondly, it’s an undocumented endpoint, meaning it can and will break at any time (and frequently does) and without warning. So use at your own risk.
As for rate limit, it’s not a documented endpoint so doesn’t have the usual rate limit rules, but just use common sense as you should be fine. For example the results are cached just like any other API endpoint, so hitting it more than once a minute for each channel you want to get a list of chatters for will be somewhat redundant and can actually start giving you inconsistent data if you poll too frequently due to potentially hitting a cache server with new data, and then a different server that may have older data.
Also you should make sure the data that you store is in compliance with the GDPR, the Developer Agreement, and any other laws on storing user data.