We are going to upgrade exchange 2007 to 2010 with 2 mail box servers(High availability) and one(ht,cas).we have 300 users.what are the license we have to purchase?

That really depends on certain factors, your best way to get answers on licensing is contact a Microsoft Licensing Specialist at http://www.microsoft.com/licensing/resources/faq.mspx or any major software VAR such as CDW, Tech Data or Ingram Micro. Most Microsoft Certfied Partners can assist with licensing questions too.MVP Exchange Server

I am neither a MVP nor a MS "License specialist", but I will tell you what I know of... You will need following licenses: - Windows 2008 64bits or R2 Enterprise license x 2 or more depends on you go for multi-role or single role Exchange servers. Standard Edition does not provide Fail-over Cluster feature, which you will need for Exchange 2010 HA deployment. - Exchange 2010 Server Standard Edition - If you need less than 5 databases, more than that you will need Enterprise Edition - 300 Exchange 2010 Standard CALs at least - if you want to use Archive or Mutli-mailbox search, you will need Enterprise CAL for that user. Don't worry about this too much, once complete the Exchange 2010 installation, EMC will calculate and display the number of Standard and Enterprise CALs for you.