Salesforce can be implemented for

Sales – Sales Cloud is used.
Service – Service cloud is used.
Marketing – Marketing cloud is used.

2. How many users do you have?

Number of active users in your Salesforce organization.
It is based on the client’s requirement.

I have supported upto 15000 users in a project.

Normally it will be from 2000 to 10000.

3. Which edition are you using? 

Enterprise edition for small client.

Unlimited edtition for bigger client.

4 What are controller extensions and how are you managing them in the project?

https://infallibletechie.com/2012/11/difference-between-standardcontroller.html