Does Apple Watch cellular works in Pakistan?

This is the list of carriers (and countries) that support the Apple Watch

https://www.apple.com/watch/cellular/

The Apple Watch shared the iPhone's phone number, so the carrier MUST support the Apple Watch, and thus must be in the above list. For this phone number sharing service, the carrier charges a monthly fee. In the U.S. the fee is $10 per month added to the iPhone bill.

If there are no carriers in your country, you would need to contact the carrier of your choice (or all of them) and ask if they are going to support the Apple Watch.