What do you have set here? see screenshot

I already have about 16 pins and would like to add more.

Screenshot didn't appear when I uploaded.

Did you try increasing the number of rows as shown in the earlier-posted screenshot?

Reply: https://support.mozilla.org/en-US/questions/1355126#answer-1453339

Image: https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2021-10-23-23-49-04-599ed0.png

Yes, drop down menu lists 4 rows as maximum.

Four rows should give you 4 x 8 = 32 shortcuts.

If you need to increase beyond that, you have to go through the back door:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste browser.newtabpage.activity-stream.topSitesRows and pause while the list is filtered

(3) Double-click the browser.newtabpage.activity-stream.topSitesRows preference to display an editing field, and change the value from 4 to 6 then press Enter or click the blue check mark button to save the change. I actually don't know how high this can go...

More info on about:config: Configuration Editor for Firefox.