TomTom is right about Exchange 2010 Standard being enough to support 1000 and even more clients but other then that there's other things you need to consider.
What you need to take into consideration if you want to go with only one Exchange 2010 STD or would you like to use DAG.
If you want to use DAG which means you can reboot one server while the other takes all the load and vice-verse and everything continues to run you need Windows 2008 R2 Enterprise and at least 2 licenses of Exchange 2010 Standard (each box would have to have all roles installed). You also need some kind of Load Balancer (hardware or software) that can direct traffic right.
Two or more Exchange Server 2010 Client Access Servers can be configured as a CAS array using NLB as long they are not also installed as Mailbox servers that are members of a Database Availability Group (DAG).
The reason is that DAG members utilize Windows Failover Clustering, which can’t co-exist with NLB.`
Exchange 2010 Enterprise doesn't give you more then amount of supported DB's (STD gives you 5, Enterprise 100 or so). Each DB can be 5TB in space so as long as 5TB x 5 is fine for you should be fine with Standard version. If you want DAG you need to buy Windows 2008 R2 Enterprise as it's uses it's clustering options.
Also you need to distinguish CAL's. There's STD CAL and ENT CAL. STD CAL covers all the basics of Exchange and it can be used (if you want to) to work with Exchange 2010 ENT. If you would like additional features (basically Archiving comes to my mind) then you would need Exchange STD CAL and Exchange ENT CAL (not just one, but both). So in other words you always stard with Exchange STD CAL whether you use Exchange 2010 STD Server or 2010 ENT Server. And the main difference is amount of DB's that are supported.
Also you perfectly fine to use Exchange 2010 STD and Exchange STD CAL + Exchange ENT CAL to get Archive functionality in Exchange 2010 STD.