IMHO and experience it comes down to how long your phone battery will last before requiring a recharge, and what size power pack you need.
If your smartphone battery tops off at, say 1,300 mAH, it is IMHO false economy to carry a larger, heavier power bank charger that holds perhaps 20,000 mAH, or more. This means that you could recharge your phone completely about 15 times, before the power bank charger was fully depleted....but why?
In my experience, my iPhone Xr last two full days, with a GPS guide, like one of the newer versions of the
Wise Pilgrim Guides running all the time. The sole other use while I am walking is as a camera. I don't use social media, and stay away from email and texts until I stop for a break or at the end of my day. My usage is a balance between going dark while on Camino, and using the smartphone as a tool, like a digital Swiss Army Knife.
I carry the smallest available external power cell that will top off my iPhone - 1,300 mAH. I think the charger pack holds 1,500 mAH worth of replacement power. That gives me from 3 - 4 days between plug in charging. But it weighs a fraction of one of the biggest most powerful charging packs.
Has ANYONE needed that sort of time in-between plug in charging on the
Camino de Santiago? I am sure these folks are among the exception rather than the rule.
As a general statement, the greater the number of mAH capacity the external power pack charger holds, the larger and heavier it will be. After water, batteries, of all formats, are about the most dense,heaviest item you would be carrying.
So, my considered advice is to obtain the smallest, lightest external power pack that will support one (but no more than two complete recharges of your smartphone.
Hope this helps.