The answer depends, I suppose, on your definition of a ‘good shakuhachi’. Most decent 1.8 length flutes can produce in the third octave, the pitches e, f#, g, g#, a, b, (or is it a# and not b?) and c. Some can produce a fourth octave d on a good day. These are all pitches above the tsu-no-meri (d#) made by opening the 2nd and 5th finger holes.
If you count the otsu no ro meri, then that’s three complete octaves minus a few pitches, notably f-natural, for example. A 7-hole 1.8 shakuhachi can produce the high f-natural too. Longer flutes, for example 2.3 or 2.4 (with a fundamental or otsu no ro of A below middle C) usually produce the third octave above otsu no ro, or the pitch ‘A’ way above the treble staff.
This means that when the otsu no ro-meri (a-flat) and otsu no ro dai meri (g) are added, one is getting into the fourth octave! Many of these ‘dai kan’ notes are not very useable in performance (for example, with the 1.8, most of the pitches above the g#), but are great for developing one’s embouchure muscles.