> Can I change my droplet IP address?
Please open up a support ticket explaining the reason you want to change your droplet's IP address.
> How many droplets can have at a time?
By default all accounts are limited to 5 droplets, however to get that limit raised you have to open up a support ticket.
> If I shut down my server and used it rarely, the money in my account will remain or debited based on monthly? Because I use this service only 3 or 4 hours a day.
You are charged even if you droplet is powered off. Your diskspace, CPU, RAM, and IP address are all reserved while your Droplet is powered off. If you want to save a Droplet for future use take a snapshot of it and then destroy it. You'll be able to create a new Droplet from the snapshot image anytime to bring it back online.