ignition is licensed by the server, not the device/windows/tagcount. I think there are some licenses that limit the client count for a reduced price, but other than that or some special mes licensing, you arent limited by devices/tags/windows.
as far as reliability, ignition has been extremely reliable for us. its not uncommon for us to go 6 or more months before having to stop the ignition service, and more often than not that was due to doing an upgrade or because of other software causing an issue.
we have quite a few different systems ranging from 1000 to 80k tags.