If you have a cellular apple watch you need a multicard in addition to the existing SIM in your phone. The multicard must be converted to an eSIM.

If the mulicard costs a fee or not depends on the ecact name of your tariff. What year do you read in parentheses behind the tariff‘s name?

I am already using my esim on my phone, if request one more esim for same number, does my exisiting esim on phone works ?

and there is no year mentioned next to plan.

If you order another SIM (Multicard, no  Ersatz-SIM), you will have two SIMs running under the same number, one functioning in the iPhone, the other in the watch when also converted to an eSIM.

Somewhere in the treaty or in the bills you should find either a year in the tariff’s name or the word “Connect”, which means that a certain amount of Multicards and Datacards are included.

When did you make the contract?

I bought my contract in Dec 2019

In this case it is very probable that it is an o2 Free M (2019), in which additional SIMs cost additional fees.

Is this one time cost or recurring charges every month? And mobile number remains the same for Apple watch?

One time cost 39,99 and then monthly recurring 4,99.

The mobile number remains the same for both.

Oops, then I am not interested. Thank you.