If you get a document that contains a header but the page number was not defined for it and you need to add the page number to your pages without deleting the previous headers, you can follow the steps below.
1- Click on the Insert tab.
2- Click on the Header.
3- Choose “Edit Header”.
4- Click on the Page Number.
5- Select the “Current Position”.
6- Choose your desired style.
Now you can see the page number was added near your header.
7- Click on the “Close Header and Footer”.