Not exactly, it’s per number of connections, so if two users are working on the same document, it counts as two connections. See the little info mouse-over thing on the page you linked to. You can also change the number of connections there, up to 200, which will adjust the price accordingly. If you need more connections, you will have to contact them.
With Collabora, the price is per user per year, according to the link posted by @devnull. If you have any further questions regarding plans and pricing, I’d suggest you contact them directly. I am sure they will be happy to help you find a plan that’s right for you.
Because of the link above from bloo it seems that the “Number of connections” is relevant for “On-premises” but not for “Cloud”. But I don’t know what “Cloud” is. Is it the version hosted by Collabora Online on AWS? I think anyone who wants to sell something should perhaps express themselves a little better.
Anyway, the best way to find out details like this is probably to ask the companies directly. Or Nextcloud Enterprise customers could maybe also ask Nextcloud GmbH, who are listed as partners with both companies.
I can’t say anything about the external hosting regarding ONLYOFFICE, as I currently don’t know of any Nextcloud instances where ONLYOFFICE is still used.
However, I have just tried a total of four Managed Nextcloud with Nextcloud Office / Collabora Online. All four providers host their Collabora Online instances themselves. I think one hoster uses a BUILT-in CODE server (demo Nextcloud).