Microsoft states that the Xbox Series X will be launched with "thousands" of games with the help of the console's backward-compatibility.
"With more than 100,000 hours of play testing already completed, thousands of games are already playable on Xbox Series X today, from the biggest blockbusters to cult classics and fan favorites," Xbox Series X Director of Program Management Jason Ronald wrote in a post. "Many of us in Team Xbox play on the Xbox Series X daily as our primary console and switching between generations is seamless. By the time we launch this holiday, the team will have spent well over 200,000 hours ensuring your game library is ready for you to jump in immediately."
Ronald told IGN that their goal is to make every single Xbox One game and hundreds of Xbox 360, plus original Xbox games compatible with Xbox Series X. The manager also mentioned that there might be some exceptions here or there due to licensing or technical issues.
Microsoft also states that the games will run better on Series X compared to the original performance. "All titles run at the peak performance that they were originally designed for, many times even higher performance than the games saw on their original launch platform, resulting in higher and more steady framerates and rendering at their maximum resolution and visual quality," Ronald said.
You can learn more here.