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?

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

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.