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

The problem is that you've written a sentence form that appears in many introductions of technical manuscripts with a different meaning: "There was a sparsity/paucity of research in this area[, motivating me to work on it]."

It's a given that more research can be performed; it's not necessary to inform the reader of this. It sounds like you mean something like the following:

The study of A has emerged as a distinct field that now merits a review.

There is now sufficient research on this active topic to review it.

This burgeoning field is now broad enough to usefully review.

An appreciable/considerable/notable amount of work has recently been published in this developing area, prompting this review.

Does this match the intended meaning?

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)

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".

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

There is a modicum of research on this subject.

"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 ...".