Currently it's not available outside of internal testing But from rumours it could start to be released to eligible devices from next week I stress this is a rumour and nothing official has been made available You will know when you see the invite to participate in the notices section on the Members App
Depends. I don't think there's a limit for how many can take part as such. But sometimes they can close beta testing once they feel enough are participating initially. It's more a limit on the eligible devices that can participate. But think carefully before participating, as the software can be buggy and sometimes things like banking apps can stop working. Once in the Beta program, coming back out of it is not easy and quite often causes more problems
Probably yes ,that was only rumours that will be start today ,is many leaks now in the social media how OneUI7 is look so I think we can expect any time from now