Hi.. Is it possible to link two accounts - one created with email id and another with mobile number using account linking feature. Use case.. User is adding a mobile number to existing account created with email id. This mobile Id is already existing as w seperate account. In all cases, user will verify with passwordless OTP

yes, it is possible. You can do manual account linking: https://supertokens.com/docs/thirdpartyemailpassword/common-customizations/account-linking/manual-account-linking

Is it not possible to link them automatically?

by manully i mean you have to write extra code in your API to link the users.