First don't focus too much on subscribers, focus on views. Subscribers naturally come the more views you get as long as you have the right call to action and all the basics down.
The successful Youtube channels out there have videos that can be searched easily and appear in the suggested videos. Ranking high in the searches means that your videos can keep gaining views potentially forever. Now to appear high in the searches, you'll have to get a lot of views, more specifically you need a lot of watch time during the first 48 hours of your upload. So unless you're one hell of a promoter, or you got a big following already, that you can get hundreds of views on your first 24 hours, then you're not taking advantage of the second day to promote your video.
So get good at promoting! My first 5 videos 2 months ago I could barley hit 40 views the first 48 hours. Then each day I did research and got a little better. Just today my recently uploaded video hit 830 views for the first time and there's still 7 hours to go.
There's an old saying, "Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime"
Promoting videos is like giving it fish. But getting your videos to rank high in the searches, that's likes teaching it to feed itself for life.