If you are charging an iPad from a PowerBank, given that the iPad battery is much larger than a mobile phone, you'll need one of the larger capacity models. Consider a PowerBank that has an advertised capacity of at least 16,000 mAh - 20,000 mAh or greater being potentially much more useful.
You should choose a PowerBank that explicitly supports USB-C with USB PD (Power Delivery) - as these have the capability to fast charge connected Apple USB devices. You'll also need a suitably powerful USB Power Adapter to recharge the PowerBank; you'll typically need a Power Adapter that itself can output 30W or greater, more powerful Power Adapters being able to recharge the PowerBank (and for that matter, your iPad) much more quickly.
When investing in a PowerBank and/or a Power Adapter, always choose a well known brand of good reputation. Cheap or no-brand power products are false economy; not only are they likely to be unreliable, but they may also lack essential safety electronics that, should a fault occur, are designed to protect connected devices (such as your expensive iPad) from damage.
Of those available, Mophie and Anker are two brands that are often recommended here. When choosing, be sure to check that the PowerBank and/or Power Adapter support USB PD - as not all do. Apple devices do not support other proprietary charging standards such as QC (Quick Charge).