As shared in the Reddit AMA on July 18, 2019
Roughly one free game per month give or take. Starting with Destiny 2 (yay!).
No free games on Stadia Base. But hey, Stadia Base gives you free access to the state-of-the-art gaming hardware in our datacenter 😉 You spend your money to buy games you want.
Follow-up question: When you say 1 per month are you talking about a growing pool with 1 added each month? Or are you talking about a single rotating free to play game per month?
Growing pool as long as you’re a Stadia subscriber!