Do you have an example of what you want to achieve exactly ?
Here is a small example of adding a linebreak in caption for pdf document. It implies using latex code in the figure caption and the caption tex package that allows that, so it is for PDF output only
Solution1 : requires escaping
---
title: Test
output:
pdf_document:
keep_tex: true
extra_dependencies: caption
---
```{r, fig.cap="caption \\linebreak with a new line"}
plot(1:10)
```
Solution2: based on bookdown format and text reference
---
title: Test
output:
bookdown::pdf_document2:
keep_tex: true
extra_dependencies: caption
---
(ref:test) A caption \linebreak with a new line
```{r, fig.cap= "(ref:test)"}
plot(1:10)
```
Also, you may be instered in this answer: https://stackoverflow.com/a/53854516/3436535
and more on subfigures: https://bookdown.org/yihui/rmarkdown-cookbook/latex-subfigure.html