If you're switching to interior LED bulbs, or just adding some in addition to what's there, the current draw could end up lower or negligibly higher, no need to upgrade wiring for that. A USB supply, say 2A at 5V, including the efficiency loss of the 5V regulator circuit in it, is going to be about 1A per. Most circuits will have an amp to spare, but if you want to add several to the same circuit then look at what that circuit powers and calculate it. Remember that the limiting factor will be the fuse first (always sized so it blows before the wiring is overtaxed) and the wiring between the battery and fuse must be considered too since it is probably the same gauge as after the fuse.
You can measure voltage droop at the amp to determine if you need a better ground or if momentary just a capacitor near it, but either way you should do the normal thing, string a dedicated low gauge wire to it with a fuse at the battery.