You need minimum 1000 subscribers and 4000 hours of watch time in order to monetize your youtube channel. Generally speaking, if you reach that threshold your channel will be approved to run adsense. However, sometimes your channel will be rejected (in the situations like using bots to manipulate views, buying subscribers, using copyrighted content, etc.) When that happens, you will have to make corrections and apply again. Sometimes, you do something wrong, knowingly or unknowingly, and you are banned by Adsense. When you are rejected multiple times, or your Adsense account has been disabled, how can you make money from your videos?
Affiliate Marketing
You can review products on your video and then share affiliate links in the description. A single product sale can bring you more money from the video than it would have normally made in one or two months.
Sponsorship
You can also approach businesses and review or promote their products and make money.