This is a very big question... 1200 virtual machines isn't small and calculating this isn't something anyone will just answer as there are many requirements which are needed to be known.
- What type of virtual machines will you be running (Linux/Windows/mixed, how many vCPU's, how much RAM, how much diskspace,...)
- Regarding storage: do you have a budget to run this on as this will define what type of storage you will end up (iSCSI, Fiber, SATA/SAS/SSD,...)
- Regarding hosts: again this depends on the budget, you can buy hosts with 256GB RAM or 1TB RAM, which type of NIC's will you use (Gbit, vs 10Gbit),...
You will have to do research on storage and hosts and match what you prefer. You could go for DELL EqualLogic with DELL servers but you could even go for HP storage with HP servers or a mix (like HP servers and NetApp storage,...)
How many VM's you can run on 1 host depends on the above, the same applies for 1 LUN (IOPS based, disk usage based),...
As you can see there are many requirements/factors you have to keep track of and I would suggest hiring a consultant for this type of job. I don't think this is something anyone here will be able to answer very easy.