This badge is rewarded to people who are the first 10 subscribers to a channel.

According to Twitch:

"The Founders Badge is a subscriber badge available exclusively to the first 10 Prime or paid subscribers of Affiliate channels and first 25 Prime or paid subscribers of Partner channels."

This badge is obtainable ONLY if you are one of the top 10 first subscribers to a channel.

https://help.twitch.tv/s/article/founders-badge