Questions about plans

Plan Enterprise €109 /per month
In this plan you can use with more than 100 employees,
I need more explanation before I buy.
I have a company with 20 employees
Do I need to install on all computers or
install only on the server and all terminals use?

You need a business plan for a company with 20 employees. And yes, you need wappler installed in each of the computers.

in this case, will I pay for each wappler installed or just a plan for 20 computers?

As I explained in the other topics you already posted - you need a license for every developer using Wappler.