x <- data.frame(a= 1:11, b =3:13, c=2:12, d=7:17, invoice = 1:11)
x =
   a  b  c  d invoice
   1  3  2  7       1
   2  4  3  8       2
   3  5  4  9       3
   4  6  5 10       4
   5  7  6 11       5
   6  8  7 12       6
   7  9  8 13       7
   8 10  9 14       8
   9 11 10 15       9
  10 12 11 16      10
  11 13 12 17      11


y <- data.frame(nr = 100:125, invoice = 1)
y$invoice[12:26] <- 2

> y
    nr invoice
   100       1
   101       1
   102       1
   103       1
   104       1
   105       1
   106       1
   107       1
   108       1
   109       1
   110       1
   111       2
   112       2
   113       2
   114       2
   115       2
   116       2
   117       2

> output
    nr invoice  letter_count
   100       1    1
   101       1    3
   102       1    2
   103       1    7
   104       1    1
   105       1    3
   106       1    2
   107       1    7
   108       1    1
   109       1    2
   110       1    7
   111       2    2
   112       2    4
   113       2    3
   114       2    8
   115       2    2
   116       2    4