I’m sort of a Hymnal guy, I own a pile of Hymnals from a large number of denominations due to researching Hymn tunes several years ago.
So if we’re talking traditional hymnody which is Hymnal-based and organ-based (as opposed to contemporary praise music) an important thing is that it’s about the range rather than the key.
The presumed starting range is C to c (for one-octave tunes) or very close to C to c, sometimes you’ll see D to d.
Then as tunes get wider in range, they push out equally upwards and downwards from that C to c starting point.
Which is why you’ll nearly always see Be Thou My Vision (SLANE) in the key of Eb, the lowest note Bb and the highest note eb, due to pushing downward a full step from C to Bb and upward a step and a half from c to eb. (It fits perfectly on a Bb whistle.)
The point is that you will encounter, in traditional Hymnals, every key from four flats to four sharps, whatever key is required to get the desired range. In the Hymnals I have in front of me now (just two) I don’t see anything beyond four flats or sharps, and those are fairly rare. The vast majority stay in the three sharps to three flats range.
Staying in the traditional whistle keys that you get by using both C# and C natural fingerings, each whistle gives two Major keys.
So if you’re staying within the four flats to four sharps range of keys you’ll need the following whistles:
Eb (for Eb Major and Ab Major) (three and four flats)
Bb (for Bb Major and Eb Major) (two and three flats)
C (for C Major and F Major) (C and one flat)
D (for D Major and G Major) (two and one sharps)
E (for E Major and A Major) (four and three sharps)
As I’ve mentioned I’ve had church gigs where I’ve only used an Eb and E.
But there’s not only the keys that whistles can play, but also the favoured range, which means bringing more whistles to the gig.
For example you can play in D Major on a D and A whistle.
G Major on a D and G whistle.
Eb Major on a Bb and Eb whistle.
And on and on.
To get all the ranges I like, and because I sometimes do encounter keys beyond four sharps or four flats at gigs, I end up bringing whistles of nearly every chromatic key to gigs.
Yes I’ve had to use the C#/Db whistle, and the B/Cb whistle, at gigs. I stuck labels on them so I can grab what I need at a gig.

(I switch the C Generation head between the C and C# bodies, the Bb Generation head between the Bb and A bodies.)