The limit of Focus Pages per site is something that’s meant to help you stay focused on the work that needs to be done so that you can more easily reach your ranking goals.
Focusing on a few Focus Pages at once is more than enough as you have to complete all task associated with Google’s top ranking factors.
Once you start ranking in the top 10 with a Focus Page, you can change the Focus Page and focus on another Focus Page.
There are big companies that have teams working with ~ 5 Landing Pages per website. There is a huge amount of work for each Focus Page to rank it in the TOP 10 of Google, so adding more of them will just make your SEO Strategy harder to execute.