and this: 
are great ways to reduce ledger lines. My instrument has a range of C4 (middle C) to G7, which is 3 octaves and a fifth. This is on the large side as armonicas go these days. The range of armonicas varies widely—there really isn't a 'standard armonica' at present—so you'll need to consider who will be playing your music and the range of their instrument.
The glasses are far enough apart that I can only reach a minor 6th in each hand at best. In the bass end (C5 and below) the glasses are a little farther apart so a perfect 5th is as wide as I can reach reliably. Some instruments have their glasses even farther apart, so again, you'll need to consider who will be playing your music and the characteristics of their instrument.
That said, multiple glasses (in each hand) are natural to the armonica and sound terrific.
'Turning the thumb under' doesn't work on the armonica, so quick scales are especially difficult. Consider using hand-over-hand if you want a scale.
Once a glass is singing, it's fairly easy to make it sing again, so trills and tremolos (of the piano variety) work well.
Notes can be sustained as long as the player can stay awake. Sustained high notes (above C6 or so?) can sound eerily like feedback, so be cautious about sustaining high glasses.
The higher glasses (C5 and up?) speak fairly quickly (with practice). But the glasses in the bottom octave speak more slowly—quick passage work in the bottom octave or so is a bad idea.
The glasses continue to ring after you release them. This effect is most pronounced in the bass end, not very apparent in the high end. (Like the harp, where the high notes die away quickly, but the bass notes ring for a long time). There is no practical way to damp an armonica glass (how do you noiselessly damp a spinning glass?)
Vibrato works well.
The armonica definitely has dynamics, but the whole instrument is on the quiet side so its dynamic range is not large.
After a glass is singing, crescendi and decrescendi work well.
I invariably amplify the armonica at performances—even just a little tasteful amplification makes a huge difference.
Write glass armonica music on a grand staff, or if the part is simple enough a single staff is fine.
This: and this: are great ways to reduce ledger lines.
'Pizzicato' of the glasses (flicking them with your fingernail) works OK from about C6 up. Below that the result is more like a pitchless thud.
A 'glissando' of the glasses just doesn't work. (Unlike the lovely effect on the harp.)
I simply won't play the glasses with any sort of mallet. The result is uninteresting (a barely audible pitched thud) and I dare not risk breaking any glasses.
The glasses are too close together to bow them with a violin bow.