Hi Shehri, we do not say "There is/are *a* plenty of chairs."
We either say, informally, "there's plenty of chairs," or more correctly, "there are plenty of chairs."
If you want to use "a", you can say "There is a lot of chairs" (informally in daily speech) or "there are a lot of chairs" (preferred, because even though "a lot" is singular, "a lot of chairs" actually designates more than one chair).
You can also say "there are chairs aplenty" but this sounds a little old-fashioned.
:)