The number of
USB-C connectors provided in the design of the charger.
USB-C is a relatively new type of connector; it has dimensions slightly larger than microUSB and a double-sided design that allows you to insert the plug in either direction. At the same time, despite their compact size, rather advanced features can be implemented in such charging ports — in particular, high power supply and many fast charging technologies. On the other hand, specifically in chargers for gadgets, connectors of this type are much less common than full-sized USB (USB A), and in smaller quantities —
one at a time, less often
two. This is due to the fact that a cable is usually used to connect rechargeable gadgets, with detachable cables it is more convenient to use ordinary USB ports, and installing such ports in a charger (even a compact one) is easy. So in fact, it makes sense to specifically look for a device with USB-C mainly if the you have such a cable already (or a gadget with a suitable plug).