How many Arrows, Bolts, or Darts (Blowgun) where commonly held in a quiver?

Maybe one or two dozen, or more depending on the situation/purpose (sport, hunting, or warfare)???

The truth is quivers are for keeping arrows, not bolts or blowgun darts.
It depends on how many arrows the archer carries in his quiver. ;)

Eric, that blowgun quiver you posted looks exactly like the one my parents brought from SE Asia, except the cap on theirs is made of a larger bamboo segment. It holds about thirty darts.

I remember asking once if they were still poisoned and they were all like "Just don't touch it."

Great, thanks for the information and pictures. This is exactly what I was looking for! :)

Do modern crossbows use short arrows, instead of bolts?

i think this is one of those things that varies so much that it is not possible to put a number on it.

I remember seeing in one of those doring kinderslay books, i think it was on pirates, and when they came to the barbary corsairs, they showed a javelin holder that had room for 3 javelins.

Crossbow quivers are genrally quite small, and crossbow bolts/war arrows quite wide. For civilian use, maybe half a dozen.
Quivers for bows are unpractival for extended wear, and are most common for horse archers or practice shooting.
The numbers of arrows varied with time and region.
In England, arrows where packed in sheafs of 24. In Scandinavia in a "tvylft" of 12. An archer was typicaly required to muster with 3 of these packs. (An english archer thus bringing twice the amount of arrows)
With each arrow half an inch wide, it stands to reason that these packs where very bulky, carrying them all at the same time would be rather awkward.

"Eastern cavalry" usually had 20-30 arrows in an "hourglass" quiver, and around 15-20 in a flat Turkish style quiver. Of course it is just rough average.
Javelin (jarid) holders usually held 3 jarids.

I can't imagine anyone carrying more than 12 to 24 arrows. I guess it depends on the size of the arrow and/or quiver being utilized.

Where "sheafs" of arrows carried in large bags or crates into a battlefield?