I have a 50GB I cloud storage plan. Do I need to subscribe this plan as I have a new phone XR (128GB) ?

As long as that 50Gb is sufficient for your needs, there is no reason to subscribe for more. And that subscription is tied to your AppleID, not any particular device. So any and all devices you own using that same AppleID and iCloud account are accessing that existing subscription storage.

It's your choice - the 50 gig allows you to backup the phone's content (documents, photos etc. ; not store purchases) to the cloud so that you have an iCloud backup to restore to if something happens to the phone (e.g. if it's stolen, becomes disabled)