LED WS2812B Input current = ± 1 µA,
As each Arduino output pin supports up to 20 mA, and that on each strip only the first LED consumes current from the pin, theoretically you can connect up to 20 / 0.001 = 20,000 Strips.
The only thing that you need to worry about from a driving perspective is the distance of the first led of a strip to the Mega. You can daisy chain strips; output of the one to input of the next one. Or you can connect each strip to a different pin.
Be aware that you will have to apply 5V at points roughly every 50 leds.
Not a lot. There are ways to extend the distance, but as is, probably anywhere between 1 and 5 meters, depending on the cable you use. If you add a TTL-chip every few meters you can extend the distance. If you use 2 max485 transceivers and cat5 UTP cable you can cover up to about 80 meters.