Server reservation

I have some questions …
Is there possibility for server reservation ?
I mean when running Asterisk on a server.
If the server fails then we have service interruption and the clients are angry you know.
There is this possibility from some other vendors and we have two servers (cluster).
Of course there is a soft functionality.
In this case if the 1-st server fails the 2-nd one continues working and people who
are talking will not even notice anything when the fail takes place.

And one more question.
How many people can have phones (ip phones, ip softphones) on the pbx Asterisk ?
How many remote users ?

Thanks in advance.