In reality, youtube income is never stable, ad rates change all the time, your better off with patreon, or twitch subs for a regular income.
But ill answer your questions using my own youtube experience.
Since I upload so slowly, my income is pretty regular, mainly relying on older videos being consistently re-watched.
1. How many video did you post to make it to that points?
- 22 videos at the moment, but obviously that depends on your type of content.
2. how much promotion did you do?
- none really, only got a twitter/tumblr/facebook page for my stuff in the last couple of years, though I would definitely recommend making a twitter if you upload slow.
3. from what you heard and/or from your experience, does it eventually will start making money, or really have to go viral before you can make money?
- Just from my experience, going viral definitely gives you a boost but it's not necessary. For my kind of content it is, posting only a few times a year means every single post must go semi-viral. Many times one video for me, needs to perform the same results that say 100 videos can do for another youtuber.