Well, it kinda depends, I think. To get it out of the way: The rules don't say. This is entirely up to the GM.
Here's how I'd do it:
If you have the time to fill out all the requisition forms, including good reasons (of up to 255 letters)*, then nothing's stopping you. Except that if you bring in 100 requisition forms to the quartermaster, he's gonna laugh and throw them away. If you come up to him and chat (unless he's busy, then you just do it as efficiently as possible) and hand him four requests, all of which are reasonable for you, then by all means try them all. Particularly the being friendly bit should help somewhat. As should having a Munitorum Manual while filling out the forms and taking your time in doing them correctly.
*Because there isn't that much space on the forms (and in the Cogitator...),
So, four or five, depending on how much time you have, if you wanna do them properly. If you rush them, I'd make up a luck roll or something to determine whether you've made a mistake on the forms: If yes, then forget rolling for requisitioning, your forms are just thrown out. The DM is busy, if you don't do it right, tough. If you keep wasting their time with wrong forms, you might even get punished! Not executed, unless you're a major spammer, but you really shouldn't have enough time or forms for that.
I'd go for "unlimited, unless there's an in-game reason". You wanna go "Ceterum censeo I need that fethin Best Craftsmanship Carapace" whenever you requisition anything? As long as your forms in order, I don't see anything but a nice plot hook there. Why does your character want that item so badly? Who notices and what do they think of it?
I'd never let such items come down purely to luck. In general, I'd always ask, before the actual requisitioning bit, "Is this something that your regiment or you yourself have access to?"
Ornamented carapace: This is for certain regiments or valorous deeds, since it marks you as special on top of being good armour. Hence, you cannot get it unless someone can commission it for you or you're in a position to order it yourself.
Needle rifle: This is a sniper's/assassin's weapon. are you going on a mission which requires it, or have you shown the right aptitudes? Then go ahead and try. The trained operator in a Super-Heavy Regiment? Yeah, no, denied, as it simply isn't a weapon the regiment has access to.
There are alternate avenues, such as trading with other regiments, but these come with their own RP hooks.
My sergeant was able to requisition fire bombs and exterminator cartridges for the entire squad, because he took some time in filling the forms out properly, with the help of a Munitorum Manual, justified it (particularly it wouldn't be only for himself) and talked to a priest to give a recommendation ("We want to bring the Emperor's Flame to the Enemy!"): Since they're all items the regiment has access to, it was possible, and with everything I did I got a nice bonus. We did get enough for the squad
So, that's mostly how I'd do it. Then again, as a GM I often just eyeball situations, which I can do well because with my groups there's generally a clear sense of how we want to play the game (we talk about that) and because I get feedback every session to make sure I get it right. It's a cooperative thing and it smooths rules like these out nicely.