If you're short of IP addresses and hosting multiple domains on a single IP requiring SSL connection to all domains, then a good solution would be to get the Multi-Domain certificate from Comodo, which is a single certificate containing more than one domain name, and that certificate is for a single server installation.
However, the use of Host Headers (which is how you can use a single IP for more than one SSL enabled domain) is not recommended for E-Commerce sites.
The multi-domain certificate can only be purchased from one of the Comodo resellers, or can apply to become one yourself.
To order the certs, you will need to first generate a Certificate Signing Request Key. This is easily done via Plesk admin UI.
Login to the Plesk Control Panel.
From the left hand menu, select 'Server'.
Click on 'Certificates'.
Click on the 'Add New Certificate' item.
Fill out the information on the page. All items noted by red asterisks must be filled in
Press the 'Request' button.
You will then be returned to the Certificates menu. From the list at the bottom of the page, click on the certificate name that you just created. Mid-way down the page, there is a box. Copy the content of this box labelled 'CSR'.
Paste the CSR into the order screen when purchasing the SSL certificate.
Shortly after ordering, the SSL certificate is emailed to you zipped along with the below CA certs:
The CA certs need to be concatenated and uploaded to Plesk as one single certificate and should follow the same order.
$ cat PositiveSSLCA.crt > CA.crt
$ cat UTNAddTrustServerCA.crt >> CA.crt
$ cat AddTrustExternalCARoot.crt >> CA.crt
Those files are available for download from Comodo's support site as well.
The order you place the contents of those files into a new file is important, and should be followed as outlined above. No blank lines should be added between the certificate contents when you copy the contents of the existing CA certs into a new file. You then provide this new file to Plesk when it asks for the CA Certificate.
In order to install the certificate, go back to the Certificate area in Plesk and upload or copy/paste the certs into the relevant areas.
Return to the Server Page and go to the IP address section. Click on the relevant IP address and apply the newly installed certificate.