Legacy personal plan has 20 page limit, but you can't change to a modern personal plan, that is not an upgrade due to the fact you lose complete customization with CSS (including page injection) and Javascript that the legacy personal plan has. Your only real choice if you need more pages is to switch to a business plan and then all coding options continue to work and you can have an unlimited number of pages. You can view a comparison chart, just scroll down and click on compare plans:
https://www.squarespace.com/pricing
FYI, You can upgrade to the less expensive business plan, be billed for the difference in price, and then downgrade back to the current personal. I don't recommend that workaround unless you are paying monthly.
I am also up against the 20 page limit, but choose to stay with my legacy plan due to a lot of code on my site that would require a business plan to keep using if on the newer personal plan.
