Get highest Vim buffer number

I have the following line in my vimrc file. This is for vimdiff:

" dp will send middle pane changes to right pane and then immediately
" update.
nnoremap dp :diffput 3 <bar> :diffupdate<cr>

However, this only works if I'm using vimdiff with three files. If I am diff'ing two files, then calling "diffput 3" gives an error message because there is no buffer #3. How do I change the 3 to "buffer of rightmost pane"? Or separate this command into two statements? Something like the following:

If (two files are being compared) Then
nnoremap dp :diffput 2 <bar> :diffupdate<cr>

ElseIf (three files are being compared) Then
nnoremap dp :diffput 3 <bar> :diffupdate<cr>
End If

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