I hv Apple Watch series 7 with type c usb cable so which adaptor (18W /20W) is required to charge I watch? thank you

any of them really

you can use an old 5Watt one to a 20watt one

it just have to have a usb-c connector

or you will have to get a new cable with an usb-a connector

or use a converter Amazon.com : usb-a to usb-c adapter

though if you wish to use fast charging you will have to get an 18/20watt with usb-c connector