I have seen this question asked a few times on different forums: and the "you can only have one throwing spear" description suits most people. However, I did see one response where the spearman involved had a set of throwing javelins (about three or four) in a quiver. There was a nice picture of such a piece of kit too.
https://rpggeek.com/thread/2210596/rega ... ng-volleys scroll down to the end of this page on the RPG geek forum for 1st edition which has the image.
While I would permit a player to throw several javelin-spears from such a quiver if they were shooting in successive Rounds of Combat, I feel that there would not be time to throw javelins more than once in the Opening Volley segment. That sort of thing narrates better with bow - equipped missile troops for me.
(I also would always permit the possibility of a wider debate in a very specific narrative however.)