Cameras usually last 1-2 hours, but you may have forgotten to charge one or something has happened. Field power is good to have!
I have two normal USB battery packs and is no expert, but have gathered some experience.
When I map it is currently with 4 cameras, so it can easily glitch to charge one. Perhaps I did not have time to do it or I am just mapping for a really long time.
The capacity of battery packs is specified in mAh, and a camera battery can usually hold 1000 - 2000 mAh. The process of transfering power to another battery takes some of that power, so you cannot count on using the full capacity. Further more (as far as I have been able to read), most batteries have their capacity specified for the 3.4 V batteries they are made of. But USB runs at 5 V. So as a rule of thumb, even a good battery will at max give you 3/4 of its capacity.
Battery packs can also discharge at different speeds. Some only at e.g. 800 mA, while others can go to 5-6 A. In general larger packs can discharge faster.
They come with a different number of output ports. 2 is most common for larger packs - but what if you have 4 cameras? I found this USB power splitter very cheaply on http://www.banggood.com/USB-2_0-A-Male-To-2-Dual-USB-Female-Jack-Y-Splitter-Hub-Power-Cord-Adapter-Cable-p-979134.html . I just got it, it works and seems to be of a pretty decent quality - really good for the price. It cannot split a data connection, but does very well with power.
Battery packs can react differently if you try to get more power from them than they like. I have one, which is specified at a 6 A discharge current via its 3 USB ports. But with 3 cameras charging it will often shut down after 5-15 minutes. There is no predictable way to get it working again, which is not good in the field. The proper behavior would be just to limit to current, but no.
That takes us to size: You can get power banks up to just above 20.000 mAh. They are big and weigh 400-500 g and if they shut down you cannot use them. So I would get 2 smaller units instead.
What about quality? You can get a big name like Sony and pay about 2-3 more for the same capacity but you are likely to get the quality (like not shutting down randomly). If you go cheap the capacity will suffer - you will get much lower capacity than specified.
I read reviews on Amazon and the like and does not go cheap, but I do not get the overpriced Sony. If I was to climb a mountain or go hiking with 2 weeks between power, yes I would get a trusted outdoor brand.