How many Pila did a Roman Soldier carry?

[Imperial Dave]

And opens a wider debate on weapons for all armies per se... was it weapon or training or something else that created the paradigm?

Re the pila thing. For me it comes back to viewing the Roman use of it holistically ie how and when it was used and how it married up with the organisation and tactics at the sharp end. Pila on their own don't win battles even if we assume they were somehow technologically superior to all other spears. They were used in conjunction with the total way the foot soldiers conducted their business. A part of the sum of the whole so to speak

[Mark G]

Absolutely- which is why the model I favour works for me. 

A thing to disable the other guys shield just as you get into his face with your killing sword . 
Kill a couple, pull back a little, repeat.

[Aetius]

You can consider the pila a heavy javelin if you wish. It's main purpose was to wound or kill the foe. Perhaps taking out the shield was a secondary function. It's up to you how this was done. This was against infantry. Against a cavalry army they would hold on to the heavy pila and use it as a spear to ward off attacks. This is how I understand the pila...

[Mark G]

How was it thrown?  Individually or the whole formation at once?

[Erpingham]

How was it thrown?  Individually or the whole formation at once?

Or even by ranks

Gaius Sulpicius, the dictator, marched against them, and is said to have used the following stratagem. He commanded those who were in the front line to discharge their javelins, and immediately crouch low; then the second, third, and fourth lines to discharge theirs, each crouching in turn so that they should not be struck by the spears thrown from the rear; then when the last line had hurled their javelins, all were to rush forward suddenly with a shout and join battle at close quarters. The hurling of so many missiles, followed by an immediate charge, would throw the enemy into confusion.
(from Appian's Gallic War)

We might note that this was a cunning plan, rather than routine, but it was clearly an option.  Does this imply in normal circumstances, the Romans couldn't throw all their javelins at once?  Certainly, the Gauls do not seem to be the screaming charge types - they sportingly sit at pilum range at let the Romans do their thing.