Whether using a SIM or eSIM, you can only have one number per SIM / eSIM, ​@Alasdair Tu. 

If you have a dual SIM phone, with eSIM capability, you might be able to have a second eSIM (from another provider).

Thanks for the reply,  helpful.

The iD Mobile eSIM Support Page https://www.idmobile.co.uk/esim   indicates ‘Double Digits - You can have two active numbers and seamlessly switch between them’.

If I were to open a new (second) plan with ID, and used this plan for the old number, could the eSIM recognise both as ‘active numbers’?

You’d need two active eSIM products, each with its own number, ​@Alasdair Tu. 

Seamless switching would depend on how the phone handles dual-SIM operation, which varies from phone to phone. 

Thanks for confirming.

Hey there ​@Alasdair Tu, as kindly advsed by ​@andewhite, you can only have one phone number per eSIM.

If you wish to buy a second iD Mobile contract with another eSIM, then if your phone is compatible, they should both work on your phone, but only one can be active at anytime.

Thanks,

Tyler