Account number

1. Yes, you can use your email address as identification for your Microsoft account. Microsoft accounts are associated with a unique email address and a password, and they do not typically have a separate account number.
2. Yes, the cloud service and OneDrive use the same Microsoft account. When you sign in to OneDrive, you are using your Microsoft account credentials, which also grant you access to other Microsoft cloud services.
3. The email you received from HKT regarding the termination of "uHub plus" cloud service is likely unrelated to your Microsoft account unless you specifically used your Microsoft account to sign up for that service. It could be a spam email, but you should verify the sender and the content before taking any action.

References:

• Common symptoms of a compromised Microsoft 365 email account
• How a Microsoft account works