The limit of Focus Pages per site is well thought and don’t need to change it.
By focusing on a few Focus Pages at once is more than enough as you have to complete all the ranking factors.
Once you ranked in the top 10 with a Focus Page, you can change the Focus Page and concentrate 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 on TOP 10 Google, so adding more of them will just make your SEO Strategy harder.