One 207=9 stations 2 through 0 (0 is 10 pulses on a rotary dial)
Add one 216=9 additional stations; you lose one of the 207 codes which becomes the transfer digit. If you pick 2 as the transfer digit, then you would have codes 3-0 (9 codes) + 21-20 (9 codes)
Add another 216=9 additional codes=27
4, 5, 6, 7, 8, 9, 0=7
21, 22, 23, 24, 25, 26, 27, 28, 29, 20=10
31, 32, 33, 34, 35, 36, 37, 38, 39, 30=10
So for 19 codes you would need one 207 + two 216s
It may be possible to mix flashing and non-flashing intercom lamps, so that if you have only one 208, you would have 3 stations with flashing lamps and the rest not flashing. I don't know because I haven't tried it or seen any documentation stating that.