What does the number following spawn rate percentage mean?

Solution 1:

The value given IIRC represents the spawning rate in the Crossroads, for example, "100%/2000" would spawn roughly once per 2000 cells. It is fixed for each Orb type.

In other lands, the situation is more complicated -- some lands have special rules which restrict Orb placement (e.g. Prairie, Caribbean, Red Rock Valley, Ocean), other lands have lots of walls where Orbs cannot spawn in or inaccessible locations where they can spawn but you cannot get there, so "once per XX cells" would not necesssarily be very meaningful anyway. Sometimes it is proportionally adjusted to the native Orb spawn rate, but there are lots of special cases. It is the best to interpret this as a relative number: an orb which says "100%/1000" will spawn twice as frequently as one which says "100%/2000".