Usually
\printbibliography[heading=bibintoc, title={Works Cited}]
would be the correct choice. But with Rmarkdown/pandoc the bibliography is produced automatically for you, so adding a \printbibliography in the code yourself produces a second bibliography.
The pandoc template located at https://github.com/jgm/pandoc-templates/blob/master/default.latex suggests that there is no way to inject options like heading=bibintoc into the \printbibliography produced by pandoc. But it is possible to set the bibliography title with
biblio-title: Works Cited
If pandoc does not sanitise the input you give there with braces, it may be possible to inject options that way
biblio-title: Works Cited, heading=bibintoc
I'm not sure if that works, but even if did, it would be an incredibly dirty trick.
With a modern biblatex version (at least v3.12 from 2019-08-17) you can add
\DeclarePrintbibliographyDefaults{heading=bibintoc}
to your preamble to make heading=bibintoc the default for all \printbibliography calls.
If that command is not available because you biblatex is too old you can add
\csletcs{blx@head@bibliography}{blx@head@bibintoc}
to your preamble. That makes the default bibliography heading bibliography have the same definition as bibintoc.
If I understand correctly it should be possible to use
output:
pdf_document:
citation_package: biblatex
biblio-style: numeric
biblatexoptions: [backend=biber, maxbibnames=999]
to have Rmarkdown/pandoc load biblatex for you instead of adding it via \usepackage[]{biblatex} and the --biblatex option.
Compare for example Using biblatex with R Markdown.
biblatex-chicagoto work with the piece of code at the end. – simohamed May 21 '20 at 19:22biblatex-chicagois a special case since it should generally be loaded via its own wrapper package\usepackage{biblatex-chicago}instead of\usepackage[style=<style>]{biblatex}. You could trycitation_package: biblatex+biblio-style: chicago-authordate/biblio-style: chicago-notes, but this may not get everything right. Maybe that's worth a feature request to the pandoc developers? – moewe May 21 '20 at 19:27