Need help no of users and capacity load

We are going to have up to 3 million users in our radius setup in the next month. At present we are using freeradius1.1.6 in linux platform and over 1 million users. we are planning to upgrade to latest version. How the performance matter with 3 million users. Please suggest interms of load balancing and capacity per server.? How many radius servers will be ideal to keep. Thanks, Rams.