Alan Jackson's book "London Termini" suggests the date of the map to be between 1893, when the platforms shown on that map as 2 and 3 (now 5 and 4 respectively) were built, and 1895 when a new island platform was added on the suburban side (what is now platform 10). The suburban platforms were then numbered A to E from west to east: B being the one leading from the Hotel Curve (later No 16) and E being what is now platform 9.
His book includes a 1905 map in which the arrival (east) side platforms (3 to 6 on the earlier map) have been renumbered from east to west, 1 to 5. Platform 4 was the only one to keep its old number, but the bay platform let into platform 4 now had its own number (3). The two departure side platforms (now 5 and 8) were still numbered 1 and 2, from west to east, duplicating two of the numbers on the arrival side.
Jackson says that this numbering anomaly continued until 1921, when the departure side and suburban platforms were renumbered consecutively with the arrival side ones, but leaving gaps in preparation for further expansion, becoming Nos 6, 10-13, 16 and 17.
Two extra suburban platforms (14 and 15) were built in 1924 on the former loco yard, which had been relocated to the famous "gasworks" site as the old yard was too small to accomodate the Gresley Pacifics (not that the new site was exactly roomy!), and platforms 7 and 8 (now 6 and 7) were created in 1926 by building over sidings in the departure side of the station.
Bay platform 3 was abolished in 1934, and platform 8 was widened in 1938 by abolishing No 9 siding.
The resignalling in 1972 led to all the platforms except Nos 1 and 2 being renumbered to close the gaps left by the abolition of No 3 and the never-did-exist No 9, resulting in most of the main line platforms (4-8) having 1 substracted from their numbers, and Platforms 10 to 17 having 2 subtracted. However, five of the suburban side platforms had a very short existence with their new numbers 11-15, as they were abolished in 1976 following the diversion of local services via the Northern City Line to Moorgate