The add-stamp tool only mounts its drag-to-select rectangle tool when the device is not detected as mobile.
Tap each page preview to toggle it into the stamp selection.
Adobe's support community states that stamping many pages at once is not a built-in Acrobat feature and requires a script. How can I stamp multiple pages of the same PDF with a batch stamp? | Community
Add StampSelect Multiple PDF Pages to Stamp on Phone
Each page preview in the add-stamp tool calls onSelectionChange on click with the page index and whether the ctrl or meta key was held down.
Tap the first page you want to stamp.
Tap additional pages to include them in the batch.
Tap a selected page again to remove it from the selection.
Verifying Mobile Page Selections
Count the highlighted pages before applying the stamp.
Compare the highlighted pages against the ones you meant to pick.
Continue with Corner Position Options or Text Customization.
Frequently Asked Questions about Select Multiple PDF Pages to Stamp on Phone
How do I select multiple pages on a phone?
Pick each thumbnail one at a time to build your batch.
Where do I find the page selection controls?
Look at the page preview thumbnails below the upload area.
How do I remove a page from the selection?
Tap the highlighted page again to deselect it.
What should I do before applying the stamp?
Zoom into an uncertain thumbnail to check its highlight state.