When it comes to partnership, viewership is a good indicator that you're taking the right steps to becoming a great streamer. This is a metric that is going to be different based on the kind of content in the stream.
Keeping in mind that context is always nuanced, so when we make statements of number expectations we may at times expect higher or lower, we also understand that users may need rough guidelines to work with. As of 2018, we expect extremely popular games or 24/7 interactive streams to average in the hundreds of viewers, long term popular games and engagement driven streams to average 50-75 viewers, variety streams and non-English or non-Spanish streams to average 20-40 viewers.
We recommend taking the time to understand concurrent viewers, and considering how to improve your channel in general to grow and keep your community.
For more information, see: How do I Become a Partner?