you really can't compare Apple(s) with ... well Microsoft(s).
It is not up to the number of users, as Mark23 already said, because postfix will be capable of handling more mails than your users will produce any day. Rather it is up to what you expect to get. Exchange can deliver you more services, workflow integration and user experience than postfix/dovecot/calendar/address book (of cause you have to pay a price!). But what do you need?
Just mail? Shared calendars or address books?
Maybe you are looking that way just to get mail. Do you need kerberized services? Are your users mainly using Mail app? Do you want the simple configuation interface that comes with OS X server instead of standard files? Then go with OS X Server. Otherwise you might consider a linux box with postfix and dovecot.
But it's all up to your requirements.
Mail Server can certainly handle this. It handles Apple's internal mail system for over 60,000 employees and contractors.
For this many users, I recommend using a combination of Mac mini Servers and Mac Pros setup as an Xsan Mail Cluster, to provide you failover redundancy if/when one of the individual servers fails or goes down.
But I am a big advocate of Apple Mail. It works well when configured properly in a setup like this and there are no CALs to rack up the costs. IMAP mail will also work on practically every device there is.
Yes, the CAL system of MS is worth a second, a third, ... thought!
One more thing: IMHO configuration of postfix is more straight (easier) than Exchange and tracing problems is by far better and transparent with postfix. This will save you expensive learning courses.
But as you said, you've already left Exchange behind...