Self-promotion is a skill that requires talent, experience and expertise. Once you've managed to establish yourself as an influencer and specialize in a particular subject, you can look at commercializing some of your content. Because initially your reach is likely to be limited, it makes sense to approach smaller companies or brands. Over time, and depending on the growth of your channel, you could target larger brands.
Successful influencers tend to be contacted by businesses and brands. Most deals are initially small. Brands may ship free products to be trialed and integrated into content. Depending on reach, brands may then offer sponsorship deals for a monetary fee. To this end, marketers do not follow tight guidelines such as a minimum number of followers.
The level of reach at which an influencer may qualify for an advertising contract depends on their field, the competition, their main platform, and an individual assessment of their persona. Online marketers often invest in influencers arbitrarily, based on subjective speculation. If someone in the industry sees great potential in you, you may have short-term success. Similarly, you may be ignored despite considerable reach.