There are various definitions/degrees of ICS. Erik's example shows each city getting about 9 tiles. Some people allow about 12 tiles per city which gives every city all the tiles it needs up until hospitals, but some would call that a 'dense build' and not ICS.
An extreme ICS allows 6-7 tiles/city.
Although by building cities closer, you will suffer more corruption due to # of cities, you don't need to spend all that time building temples, cathedrals, courthouses, etc., like you would need to do for the large cities, and can focus more on military. The denser you build cities, the less you would need infrastructure. ICS is great for a quick conquest/culture win or domination on the larger maps with lots of land where you have room to set up the settler flood. It usually won't do well for a quick spaceship win, but a 'little dense build' of 15 tiles/city may be good for spaceship, though. ICS is great for unit support and commerce, but it won't keep up with production when factories come into play.
With a denser build you can use every tile so much sooner in the game, that hospitals aren't a big deal and the game should already be basically decided. With ICS, hospitals usually would just add specialists. I can get way more specialists by doing an extreme ICS and irrigating everything than by building hospitals.
With an extreme ICS you could create a settler flood and have 100+ cities by 10 A.D. If you set 100 cities to all build a horsemen, then I'm sure you could figure out a minimum number of horsemen you would have 30 turns later. ICS cities don't build much infrastructure, because the cities won't need them, unless 1.) quick culture win, then rush temples, libraries in every city or 2.) early conquest, then barracks in the cities that will build military units.
What I do with my extreme ICS/settler flood is to mine all grassland (irrigate bonus resources) that is within 5-10 tiles of the capital (distance would vary by map size), and then irrigate EVERYTHING else.
There have been a couple articles you might want to search for (both written several months ago). One was by Aeson that it is the Strategy Articles forum that had 'expansionist' 'Chariot' and 'ICS' in the title. Another one was by JuicyCivNewbie that is in the strategy & Tips forum titled something like "How to get 100+ cities by 10 A.D.".
But the best method would be to do a 'Fringe ICS', that I saw Cartouche Bee mention a while ago in the HoF thread, and I really should try this method sometime. With the Fringe ICS, you have your cities by your capital (the optimal # of cities), spaced nicely apart so they can stay big and productive with libraries, universities, etc. Then when you get to the higher corrupt cities, ICS like crazy and irrigate everything for the specialists. (specialists are immune to corruption).
