checkboxGroupInput - set minimum and maximum number of selections - ticks

You can use a little JavaScript to do it:

## In a file named 'js4checkbox.js' in your app folder :
$(document).ready(function(){
  $('input[name=SelecetedVars]').on('click', function(event){
    if($('input[name=SelecetedVars]:checked').length > 3){
      $(this).prop('checked', false);
    }
  });
  $('input[name=SelecetedVars]').on('click', function(event){
    if($('input[name=SelecetedVars]:checked').length == 0){
      $(this).prop('checked', true);
    }
  });
});

And in your ui add:

fluidPage(
  includeScript(path = "js4checkbox.js"),
  ...
)

I don't know why but it doesn't work well in the RStudio Viewer so open it in your browser.

For the JavaScript code see this post

You can do something like this:

library(shiny)

my_min <- 1
my_max <- 3

ui <- fluidPage(
  sidebarLayout(
    sidebarPanel(
      checkboxGroupInput("SelecetedVars", "MyList:",paste0("a",1:5), selected = "a1")
    ),
    mainPanel(textOutput("Selected"))
  )
)

server <- function(input,output,session) {

  output$Selected <- renderText({
    paste(input$SelecetedVars,collapse=",")
  })

  observe({
    if(length(input$SelecetedVars) > my_max){
      updateCheckboxGroupInput(session, "SelecetedVars", selected= tail(input$SelecetedVars,my_max))
    }
    if(length(input$SelecetedVars) < my_min){
      updateCheckboxGroupInput(session, "SelecetedVars", selected= "a1")
    }
  })
}

shinyApp(ui = ui, server = server)