It's not really likely for someone starting out to be making a few million in medicine these days, sorry. The barriers to entry for starting a business and practice overhead expenses are higher these days, student loans impinge one's ability to obtain working capital, insurance companies severely hamper what most patients can be charged, and just try to get a small business loan in today's financial turmoil. So no, I can say with 99% certainty that nobody on this thread is going to earn anything like a million (let alone a few million) from their physician salary or partnership share. Folks who got into the business when the time was right might, but our generation will not do as well.
But in terms of your question, if you have multiple practices, that implies you have multiple people working under you -- if each generates more income than their salary plus expenses, the boss makes the surplus. That's what I described as leverage above -- you make money off the folks working under you. The concept is good, but in today's medical landscape, it's going to be unlikely to set up.