vCenter 2.5 has a hardlimit of 200 hosts and 2000 guests, this has however been raised to 3000 guests in vCenter 4. and this can grow to 30000 guest with the ability to create Linked vCenter instances.
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Tom Howarth VCP / vExpert
VMware Communities User Moderator
Blog: www.planetvm.net
Contributing author for the upcoming book "[VMware vSphere and Virtual Infrastructure Security: Securing ESX and the Virtual Environment|http://my.safaribooksonline.com/9780136083214]”. Currently available on roughcuts