there is a total of 9 generations or "Ages" according to the wikia.
these were effective just walking cockpits according to the wikia and weren't outfitted with weapons. though we have seen no examples of these considering that the Cockpit Ejection System was developed we can imagine the normal Knightmare Cockpits with legs
this generation came about after the creation of several common systemd found in Knightmares like Factsphere Sensor and Landspinner Propulsion System. this allowed for the early Knightmare Frame prototypes to be developed. however these new technologies behaved erratically so the research was outsourced to a Britannian Army Special Division, the 'Special Dispatch Guidance System Division', and a private group, the Ashford Foundation. it was also in this generation that the term "Knightmare Frame" was born
The bipedal weapon was nicknamed 'Knightmare' by the Army but its non-offensive equipment was referred to as 'Frame' by civilians. The union of those two terms gave birth to the name 'Knightmare Frame'.
research done by the Special Dispatch Guidance System Division and the Ashford Foundation lead to the development of combat-effective Knightmares. this also pushed the value of Sakuradite due to it being paramount in making the Knightmare Frames perform to expectations.
the 3rd Generation is infact the earliest generation we see in the shown with the only one being the prototype Ganymede and its test pilot was Marianne. it was this which quickly made famous and being granted knighthood before she was married. however her assassination lead to the Ashford Foundation retiring from the Knightmare R&D
the RPI-11 Glasgow is the most commonly shown 4th gen Knightmare and was the first mass production unit for the platform. it was also the RPI-11 Glasgow which allowed the Britannian Empire to easily dominate Japan during the invasion in 2010 and became their baseline unit for the military.
while it later becomes obsolete it is still used by the Knight police and other countries/factions use the Glasgow as their base for Knightmare Development like the Burai which is produced by the Kyoto House for the Black Knights and the Japan Liberation Front
Due to Britannia's success in Japan with the Glagow this generation saw the development of Anti-Knightmare weapons leading to create the RPI-13 Sutherland design to take on other Knightmares.
the 6th generation showed no real improvement with Knightmares and thus is referred to as the "Missing Generation". despite this and it's seemingly overwhelming power the Gawain is a 6th generation model privately developed by Schneizel el Britannia and later (much to Lloyd Asplund's annoyance) perfected by Rakshata Chawla. also despite it being seen used more often by a 7th Gen Prototype the Gawain was actually the first Knightmare to use the Float System
the Lancelot was the first significant innovation in the technology and lead to the development of the Vincent series of mass production unit. however the Lancelot still remained as a unique unit due to the development of experimental technologies not found in the Vincent such as the BlazeLuminous shields
the 8th generation was primarily seen during the second season with advancements in Knightmares being lead by the heads of Camelot Division, Lloyd Asplund and Cécile Croomy for Britannia and by Rakshata Chawla for the Black Knights. the Knights of the round all use 8th Gen Knightmares as such prior to this (and possibly prior to Susaku's appointment as a Knight of the Round) if they did use Knightmares they were probably 5th Generation, possibly Gloucesters considering Cornelia and her bodyguards use of them.
as of the end of the second season this is the pinnacle of Knightmare Development with there only being 2 units, the Lancelot Albion and the Guren S.E.I.T.E.N., both of these making use of the improved Float System called the Energy Wing.
It is possible that there may be a mass production series of the 9th generation or even more generations after thus the years following the Zero Requiem however until the release of Code Geass: Lelouch of the Resurrection we wont know for sure