how many 2x8's for center beam
You are asking for specific design from a do it yourself internet chat forum. This type of question is pretty common on this forum, and often comes from folks who are not confident they can do the design themselves, but do not want to pay to hire an engineer or architect to perform the design. Unfortunately design of a major supporting element for an inhabitable structure carries serious risk, since collapse of the garage has the potential for injury to people.
The professionals on this forum cannot legally give you explicit design advice without a contract, plus they need to be licensed in your jurisdiction. That leaves you with unlicensed non-professionals who may be willing to offer specific design information without having visited your site to determine specific conditions. Are you ready to accept and act on such advice? Is your building inspector willing to grant you a permit based on advice from a non-professional on an internet chat forum?
My suggestion is to go to a local lumberyard (NOT a big box store), show them photos and a plan for your garage, and allow them to include the design of the beam in the price for the lumber. One way or another, you are likely to end up paying for structural design of the beam, and getting the lumberyard to roll it up in the price of the lumber (assuming the lumberyard has an on call engineer) could easily be the lowest cost way to get it done.