22 frets on banjos

MuseScore doesn't have a 5-string Banjo at all as far as I can see.
So you must have added that 5th string. Workaround: Rightclick into the statt > staff properties > string data and there add those frets

To add it to MuseScore: what would the tuning be?

See also #275757: Banjo 5th string problem, and proposed fix, I guess this should be dealt with there.

Right-click on the staff -> Staff Properties -> "Edit String Data" -> Edit "Number of frets" to 22 -> Ok

So, it's not an issue, but "by design". The default - the most common - is 19 frets ( but you can edit the number as wished.)

"MuseScore doesn't have a 5-string Banjo at all as far as I can see."

??

I found none. There are 3 different banjos, all 4 strings. No 5-string Tablature either.

? Don't understand. 5-string banjo is the "standard", the most common banjo. And Tab 5-str. is here, as always.

Hmm, not sure what I've been looking at then, now I see 5 strings indeed, before I saw only 4?

Ah, I looked at the Tenor Banjo and the Irish Tenor Banjo, both 4 strings.
And as per Wikipedia a Plectrum Banjo also has only 4 strings, but indeed 22 frets, vs. 17 or 19 for the Tenor Banjo

So I guess we could add a Plectrum Banjo

It's a not a workaround. It's precisely a feature for edit the number of frets. There is litteraly thousands of plucked instruments with various tunings, number of frets etc.
If they all had to be installed in MuseScore, the space would soon run out.

Actually, most 5-string banjos are 22 fret, a few are 24. Old-time style 5-string banjos (with a scooped neck) are usually 17. I'm not sure I've ever seem a 19-fret 5-string. The default should be changed to 22.