Is there an antonym for "paucity" that means not scarce and not necessarily but possibly enough?

How could you describe this amount of fuel?

enough

OED

A. adj. (determiner).

I. As much or as many as required or wanted; sufficient in quantity or number.

1934 Butler (Georgia) Herald 16 Aug. 2/6 Heat and cold are cruel things, but man has intelligence enough to overcome them.

2016 Church Times 9 Dec. 11/2 The Archbishop of Mosul, Mor Nicodemus Daoud Sharaf, and the Archbishop of St Matthew's..were denied visas..because they did not have enough money.

substantial or significant means more than a little but not necessarily enough. non-trivial is an even more direct antonym of 'sparse/ity' or 'little' or 'paucity', but slightly arch.

But I do not agree with your highway example for 'more than a little but not necessarily enough'. If I think I have enough fuel, even if it's only a little, I'll stay on. If I think I don't have enough fuel, it's not safe to continue, regardless of how small or large that amount is. Plus, with GPS nav I do know both how far the next exit is and whether there's a restaurant -- and maybe even if it's a good one :-)

I would propose limited. Saying,

There is a limited amount of research on this topic.

would imply that, although there is some research that has been done and published on the topic, there isn't very much. There being a limited amount of research doesn't pass judgment on the quality of the research that has been done, and it's entirely possible that, even with a limited amount, some important conclusions can be drawn and a decent understanding of the overall topic can be cobbled together.

This seems to fit your intended meaning quite well.

That said, I would generally opt to go with Chemomechanics's suggestions to reword your sentence entirely, obviating the need for a "single-word request".

A plain some, as JeffUK suggested in a comment, would fit very well. It acknowledges existence without qualification, perhaps with a hint that it is not complete: "While there is some [existing] research on this topic more work needs to be done."

"Some" can be accompanied to further describe the nature or amount of the research. Some of these examples are from other answers: some initial or basic research, some substantial or considerable research. If it is unsystematic, one could call it some scattered or haphazard research.

A small, uncoordinated effort one could call "a smattering of research".

If you wanted to fill in the blank exactly as written, you could go with

There is a modicum of research on this subject.

I would say, "the research on this topic is inexhaustive", and leave it as a matter of insinuation that I'm not rebuking the research for not being good enough, only that it has not exhausted all the research avenues.

Another possibility is "there is foundational research on this topic". This conveys the idea that the groundwork has been adequately laid, but the topic isn't closed; we could add a qualifier to topic: "there exists foundational research about this open topic".

Possibly "ample" works for you.

As per The American Heritage Dictionary

adj. Of large or great size, amount, extent, or capacity: synonym: spacious.
adj. Fully sufficient to meet a need or purpose: synonym: plentiful.

Here, we'd be wanting the 2nd definition. To fit your sentence structure, it would be:

There is an ample amount of research.

sufficiency

the state of being sufficient (MW)

adequate body of literature on this topic (to justify this survey)

of a level of quality that meets one's needs or standards (MW)

"Modicum" expresses that there is a quantity, usually of a desired thing, and implies that more may be desirable; essentially it means "some" or a small amount. For example, a modicum of research has been done in this area.

It may be better to use the more informative sentence that you put after your "what I am trying to say is ...".

The antonym of paucity is abundance, per Oxford American Writer’s Thesaurus.

There is an abundance of research…