I assume you are in the UK? I am and on EE, in the UK.
My phone has an EE sim, my watch has an Esim and connected to EE on a separate plan
Both devices have separate numbers. These numbers are independent of each other..BUT..
If I'm away from my phone, wearing my watch, with mobile data enabled on my watch, and someone calls my mobile phone number, my watch receives the call too. I can take the call using my watch.
IF some rings my watch number, my watch will obviously receive that call. But not the phone.
When you are wearing your watch away from your phone, as long as you have mobile data enabled its like you are wearing your phone.
When the watch and phone are together, I have Bluetooth on both devices and they are connected together. You should have WIFI enabled too for full connectivity.
Each device has a separate data plan. So each device has a separate number.
The EE watch plans are usually around £7 for 10GB. Unless you get a good deal.
I believe its practice to have the phone and watch with the same provider too.
I hope this helps.