Hello every body 
I would like to find a person interested in customizing the Mail App to the following.
Mail App that requires a bridge for auto-login:
I would recommend you reach out to Nextcloud GmbH official support - don’t expect serious offers from volunteers community…
btw AIO might be not the best choice for such big system 
Thank you for the reply,
I don’t want a heavy system - I want only the files, office, Mail.
What server specs you recommended to handle 5000 users? (It’s low, i except even millions of users)
Do you recommend good server specs? Provider? and nextcloud setup?
You said AIO is not the best for big systems, what do you mean? to install it manually?
Nextcloud GmBH is very expensive per user.
Thanks, looking forward
You probably need to use several servers to balance the load, and then also use a database and storage cluster. That is not trivial.
If you can split up the setups to have just units with a few hundreds of users, then you can deploy such setups on a single server.
In case of millions of users, you can perhaps get some reductions.
this installation method is intended mainly to small installations. Simplicity of the installation the main goal not customization and sizing
everything has it’s price. You may negotiate lower prices… but talking about 5000k users or more your business case sounds important enough to spend some money on qualified support. Feel free to find another professional but think long term - cheap support often ends in catastrophic results.
What is is with everyone wanting to scale horizontally ? That is not neccessary for 5k users at all.
The user number is pretty useless anyways, as what really counts is the activity of the users = the number of http requests per second. But if those 5k users are not constantly hammering the server vertical scaling will be easily enough !
Just get a big AMD Epic Server (Colocation with own hardware is cheaper in the long run, but you can of course also just get a dedicated server from some hosting provider, like the AX161 from Hetzner), throw some fast storage in for the database use ZFS for HDD storage and your are set for 5k and FAR beyond users of average activity.
You really don’t need horizontal scaling until:
The actually important question is: What is the budget for the monthly costs for the server ?
They had some deployment recommendations in the past, and they were given rough number of users. Even if you start now with a deployment that just works like this on a single server, you perhaps want to expand the services (to more users, or if the service is popular it is used more often or more apps are used etc.) and this makes you more flexible on the long run.
Do you have some numbers, how many users can be managed on such a hardware and which apps are used (with online office and online conferences?)?