There's a lot of people on Discord (around 200 million monthly active users) but it's still the smallest out of all the major messaging services that support group chats. For example, Telegram has over double the number of users, and WhatsApp has 10x the users.
For open source projects in particular, something that integrates with Github and Gitlab login (like Gitter which is now powered by Matrix) is a better choice, as developers are practically guaranteed to have one of those accounts.