what product/plan is right for me?

Hi

I am developing a visitor management app and I need to capture the visitors' signature on a tablet and feed it into my app.

This is necessary for instance when you are instructed on the safety rules you have to follow while inside a factory let's say.

This is an SaaS app and I need to know the following:

1. Which solution and which plan should I use?
2. Let's say I have 100 clients but they will all use my app. Do I need to purchase a plan for each of the clients or it is enough to only buy one.

Thank you and bear in mind I have no experience with adobe sign solution so I may need a more detailed answer.