I have bought my iPhone second hand. It had no sim. I’ve purchased and input a new sim. How do I know my phone number?

See -> If you've forgotten your Apple ID email address or phone number

If you've forgotten your Apple ID email address or phone number – Apple Support (UK)

Who is my carrier:

Go to -> Settings -> Mobile network -> scroll to Network selection you can see who your carrier is.

Contact your cellular carrier (the one who gave you the SIM card) about that.

Or, if you mean how to input/change your iPhone number: Settings>Phone...

Regards

Giulio