where can I find my licence key number?

What license ?

`~`

If you upgraded to Windows 10 when free or you bought a computer with Windows 10 installed you do not need a key as your computer should be activated.

To check if your computer is activated;
Right click Start, click Command Prompt(Admin)
enter slmgr /xpr  Then hit the Enter key
It will tell you when it expires. 

Depending on how Windows 10 was configured on your key, you might not have or need a key. See:

https://www.groovypost.com/howto/find-your-windows-10-product-key/

When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key.

Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate. You will prompted to enter a product key a couple times through the installation, click I don't have a key and Do this later.

There is no need to know or get a new key, but if you have issues with Digital Licensing or the Activation Troubleshooter, you can utilize an existing Windows 7 or Windows 8/8.1 product key you already ownto reactivate if necessary. or use the reset function in Windows 10.