Only one site or many?

If you have one site which you managed with web hosting, will you still need more sites? Why would anyone stick to just one site? And some are focusing on many? Here are my evaluations:

One site only: If only one site, you will have the main focus on it. You can promote it anywhere you want, especially some SEO skills, but revenue and traffic will be less generated depends on your niche.

Many sites: If many sites, advantage is when you do SEO on them, you have a chance to get more traffic and earnings in your ad revenue site like Google Adsense, but the thing is, you cannot focus on one site to manage and update it.