Anyone making music should be aware of the "circle of fifths" (or if you go around the circle backward, the "circle of fourths").

In the major key of C, there are no sharps or flats.
In the major key of G, there is one sharp.
In the major key of D, there are two sharps.

The "rememberer" I was taught as a 4th grader was

"Go Down And Eat Breakfast"

G: 1#, D: 2#, A: 3#, E: 4#, B: 5# in the "key signature" on the music score.

Going the "other way" into "flat keys" -

Fat Boys Eat Apple Dumplings

F: 1 flat, Bb: 2 flats, Eb: 3 flats, Ab: 4 flats, Db: 5 flats

If the sequence for the "keys with flats" is reversed, and a couple of keys are added to fill in the few keys with "ridiculously many sharps or flats" a well known sequence emerges that, if laid out on a circle repeats itself (sort of) indefinitely.

Db = 5 flats in the "key signature"
Ab = 4 flats
Eb = 3 flats
Bb = 2 flats
F : := 1 flat
C : := no sharps or flats
G : := 1 sharp
D : := 2 sharps
A : := 3 sharps
E : := 4 sharps
B : := 5 sharps
F# = 6 sharps is enharmonic with Gb (6 flats)
C# = 7 sharps is enharmonic with Db (5 flats)
G# = 8 sharps is enharmonic with Ab (4 flats)

Note that the bottom few "keys" are the same as the top few (enharmonically) so you've "closed the circle."

If the "keys" of the harps are listed in this order, to pick the harp key for "cross harp" playing of a tune in any key, just "move up one" (i.e. pick the harp key with one flat more, or one sharp fewer, than what's shown for the "key signature of the tune."

A quick Google for "circle of fifths" should find many sites with pretty pictures of the "key progressions" laid out in circles that may make things clearer. Just pick whichever one is best suited to your personal fancy and learn the sequence for the range of keys that interest you.

Note that the key of C Major has the same "key signature" (no sharps or flats) as A Minor. The "circle" for Minor keys is identical to the one for Major keys, just starting in a different place.

John