On the topic of number of thrown missiles to carry... Factors:
1.) Weight. Most javelins and darts are fairly light, especially those with hardened wood points or bone instead of metal. So that typically isn't your major concern. But there are exceptions. The pila and its close cousins are all quite heavy as far as javelins go,which is obvious given the amount of iron used in thier construction, and they could not be thrown terribly far.
2.) How to carry them. Typically if you had javelins you would carry them in your shield hand, ready to throw. So how big is your hand? I would suggest that up to seven missiles is plausible. Beyond that, you would require some sort of Gumbi-like extensible hands to wrap around them. If you had some kind of quiver or bag you could carry more, but despite what native M&B says I've not seen such a thing.
3.) Role/requirement. Javelins were never meant to be the primary weapon of your primary force. They were either used by skirmishers, who would withdraw if their brief barrage didn't inflict enough casualties/disarray to close with swords, or they were used by close order infantry just before the lines met. Therefore there are practical limits as to how many you would WANT to carry.
I would suggest that Pila stay at 2. That "heavy javelins" be renamed simply "javelins", and get 4 throws, with a slight bump upward in melee skill for peltasts and others who depend heavily on their thrown weapons. That native "javelins" be removed completely unless somebody can show relevant images of troops with bags of ammo on their backs. Finally, that a new weapon be made, "darts" (very short javelins with fletching), which get 7 throws, and do about 24 damage. The Romans might get plumbatae (as a dart, with added lead weight), which have 5 throws and do 30 damage.