Do I need to buy multiple SSL on the same server?

for multiple domains

[jimpoo]

I'm going to build ten widget websites on my server, I'm confused about SSL issue.
So, do I need to buy 10 SSL certificates for each of my sites? Or just buy one SSL certificate for the server, then share by the 10 sites?

Also, when using API to handle payment datas between my server and the payment gateway server, does the gateway verification bease on IP or domain name or server?

For example, there are two of my sites:
1. widget1.com
2. widget2.com

When shopper click to checkout page, I'd like to use following:
1. [widget1.com...]
2. [widget2.com...]
Then, the checkout.php will use the same API payment class in the server to exchange data with the payment gateway, for example, authorize.net.

Anyone has similar experience?
Thank you!

[Conard]

When shopper click to checkout page, I'd like to use following:
1. [widget1.com...]
2. [widget2.com...]
Then, the checkout.php will use the same API payment class in the server to exchange data with the payment gateway, for example, authorize.net.

If that is what you want then you will need 10 seperate certs. They are issued by domain name.