I think the parameter slide_levelmay be what you are looking for:
slide_level , . , , - . , _
, slide_level: 2:
---
title: "Example"
output:
beamer_presentation:
slide_level: 2
theme: Dresden
---

,
This is an R Markdown presentation. Markdown is a simple formatting
syntax for authoring HTML, PDF, and MS Word documents. For more
details on using R Markdown see <http://rmarkdown.rstudio.com>.
When you click the **Knit** button a document will be generated that
includes both content as well as the output of any embedded R code
chunks within the document.
, , :
---
title: "Example"
output:
beamer_presentation:
slide_level: 3
theme: Dresden
---
# Section 1
#
#
#
#
#
#
#
#
#
# Section 2
#
#
#
:
