How many videos do you need to upload before you see the analytics on when the best time to upload is?
Not sure to be honest. Because I've seen some channels with thousands of subscribers and views still not have access to best time to upload analytics.
You can do half the work yourself though. Go into your demographics analytics and see which country is your main audience country. Then look at their age range.
If your demographic are teenages from America, it's safe to assume uploading during a week day means you'll be dealing with school and homework, so uploading on the weekend would be advised.
If your prime demographic is say India. India and America are completely different time zones, soo you'd want to be adjusting for the best time in India... maybe before work, or after work. Most adults tend to watch content either on lunch break, or more likely after finishing work. So if your prime demographic is India... try to publish maybe around 4pm in India's time zone.
I have a growing demographic in Portugal, so I try to hit at times when it's the evening in Portugal as well.
The "best time to upload" is still a very new, beta feature, but you can do a lot of the 1 and 2 yourself by knowing who your audience is, where they live, how old they are, and figure out what kind of lifestyle they might live, study/work and experiment with differant times that seem to fit those time periods. "Best time to upload" simply does that work for you, but you can still do that yourself.
edited to add
Also I had a video get an extra 12k views 3 months after publishing... so the actual time you release a video only matters to it's out of the gate performance, but doesn't mean it won't perform better months after publishing. Do the basic common sense work for out of the gate, but don't worry too much about when you publish, because once it's out, that's when it gets discovered, and discovery of a video isn't dictated by the time of the day it's released.