Is it Possible to use a Single Certificate for a site which has many different domains

Well,

The key point is that you can only assign a single certificate per Input Endpoint for Windows Azure service deployment. That said, you must have a single certificate for all the domains you want to serve. The way to achieve that is to use Subject Alternate Name (or SAN) Certificate.

Here is a (non-complete) list of some SAN SSL providers:

• Verisign
• ClickSSL
• DigiCert

UPDATE based on the comments

Is it mean that, if I have abc.cloudapp.net which is pointed by abc.com, abc1.com, abc2.com then only 1 certificate is enough

Yes, however this one certificate will be SAN certificate (please follow that link and read more about SAN Certs) and will have all that domains in its SAN (Subject Alternate Name) property. That certificate is typically more expensive that the regular SSL Certs.

Is there is any legal issue of using a single certificate for many domains? Whether CA will allow this?

NO. There is no issue. That is the purpose of SAN Certificates. Please follow the provided links and read more about SAN Certificates.

Yes, You can secure a different domains using single server SSL certificate. In order to secure multiple domains using single server certificate, that certificate must SAN SSL, which allows up to 100 multiple domain SSL security on single one.

Below enlisted are the list of the most trusted SAN SSL certificate provider, but at low price:

-> verisign -> rapidsslonline -> digicert