Perhaps you could clarify by what you mean by "portable charger" as the language is often rather imprecise these days. I've heard of the word "charger" being used for cables, AC power adapters (where smaller ones are often referred to as "portable", or USB (battery) power banks (that provide self-contained power).
The iPad mini 5th gen is the last iPad mini to use Apple's proprietary Lightning port. As far as I can tell, the iPad mini 5th gen came with a Lightning to USB-A cable and a 12W USB-A power adapter with Apple's modular "duckhead". Here's photos of the cable and the power adapter:
There is a certain amount of flexibility, but you can also use Apple's newer USB-C to Lightning cable with a USB-C power source, but that might just make things too complicated.
If you mean a USB power bank (to have power without direct access to AC power) there is just way too much out there. I've successfully used smaller ones to power an iPad mini 4, but they may not last very long and may have limited power output (often 5W). My preference is for something with at least an 8000 mAh battery, and this will usually be stated somewhere in the specifications. This is one I personally use, but it's discontinued. But I'd recommend something similar. This one has two USB-A ports and it operates just like plugging into a regular USB power adapter. The only caveat with most USB power banks is that they typically stop providing power after a while as some sort of power saving mechanism and need to be restarted by pressing a button that both restarts the power flow and lights up charge level indicator lights.