Hi,
From doing the maths there your looking at 12 x 24(logical processors = vCpu's essentially) = 288 so if you have 225 vm's at present (with 1 vCPU each) that leaves you with 63 logical processor/vCPU's left before you over-subscribe.
Just so you know we have a quite a large VDI environment (running XenDesktop 5.6 / vSphere 5.0 u2) and over-subscribe to the order of 3:1 - so in essence if 300 vm's running 1 vCPU: 100 vCPU's - and the performance is fine.
Key thing to make sure is:
i) Your storage sub-system is adequately specc'ed for IOPS
ii) You reboot idle VDI's in staggered sequences to lessen the load.
The 3:1 ratio above is a "middle ground" in our environment, for heavy duty users you might want a 2:1
Many tx