Ipad external storage support
how much external storage can be use in ipad 9 generation through pendrive
For both practical and cost purposes, you are unlikely to find a Pendrive that has more storage than your iPad can use.
You can find guidance about connecting external storage to your iPad here:
Connect external storage devices to iPad - Apple Support
For iPad9, to connect your Pendrive or other USB storage to your iPad, you will require an Apple Lightning to USB 3 Camera Adapter: