\version "2.21.0"

\layout {
  \context {
    \Score
    \remove "Bar_number_engraver"
    \override BarNumber.self-alignment-X = #LEFT
    \override BarNumber.font-size = #10
    \RemoveEmptyStaves
  }
  \context {
    \StaffGroup
    \consists "Bar_number_engraver"
  }
}

<<
  \new StaffGroup {c'''1 \break 1}
  \new StaffGroup {c'  1 \break R1}
>>