In the 2014 movie Guardians of the Galaxy, Groot is able to be regrown after being completely destroyed. Why couldn't the Guardians of the Galaxy members just plant a hundred Groots from his sprouts and build an army? Is there any explanation about his regeneration abilities in the comics?
The Comics are relatively ambiguous about this, as so far there has never been a depicted attempt to grow multiple Groots.
In the comics, however, Groot is depicted as occupying a consciousness as long as a part of him remains, and it is inferred cultivated...
In an earlier issue, on another occasion when Groot is disassembled, Rocket is shown tending to the sapling. We can infer from this perhaps that Groots consciousness is possibly of a nebulous nature, but he can only inhabit a form of his body that meets a certain threshold.
None of this is explicitly confirmed, only inferred through the narrative of the more recent run of comics.
It's necessary to bear in mind that the Marvel Universe explicitly embraces all aspects of mysticism and 'magic' (read Chris Claremonts run of Dr Strange if you're doubtful of the validity of such an aspect of Groot's consciousness), so this is by no means far fetched or in any way outlandish a claim.
Marvel has long ceased bothered elaborating on the more mystical elements of its characters in favour of engaging narratives that assume a certain level of acceptance from its readers; as opposed to validating and cataloging every aspect of its scientific lore or plausibility, as the Silver age comics were prone to...
To clarify: there's no overt explanation for the exact nature of Groot's re-incarnative abilities as he is a character that has been elaborated on in the Post-Dark age of comics, where the sustained Marvel Style is more of playfulness and less of cumbersome fidelity.
I don't think the Guardians can make an army of Groots for two reasons (both from the comics):
Groot has been destroyed and regrown at least three times. Each time only one Groot is regrown from a sapling, so it appears that his mind can only inhabit one body.
Groot is believed to be the last of his race in the Marvel Universe. If he were able to reproduce, it stands to reason that he probably would.
As I said, these are both from the Marvel comics universe, so it doesn't necessarily hold for the films. Yondu is one character that has already been almost completely re-imagined for the Marvel Cinematic Universe, so there's no reason they can't make a few alterations to other characters as well.
If it were possible to grow multiple Groots, Rocket probably wouldn't because they'd be too difficult to manage in a group. And I suspect quite clumsy in a group scenario, so there could also be risk of copies of him being stolen by enemy forces for their own use, and Groot is quite powerful, so this would be bad.
But yeah, there's no good explanation of how Groot's consciousness works. We don't even know for sure if his remains require water and soul to regrow or if Rocket just does that for kicks (joking) because he's partially crazy.