Stagecoach have the largest fleet, about 240 are left, the majority of which are at West Ham, with as little as five left at Carford, though that garage should get some more for the short term to run the 178 from next week. Stagecoach had roughly 700 Tridents at their peak, I however stand to be corrected on that figure.
London United is down to roughly 25-30, most are at Tolworth but about 10 are at Hounslow, and tiny handfuls are left at PK (220 spares) and FW, mostly used on schools and 371. LUs fleet reached about 350, all with ALX400 bodywork with long and short wheelbases.
CT Plus has about 13 East Lancs bodied Tridents and these are on their way out as new Enviro400 Citys replace them. They totalled 14 if I remember correctly.
Abellio still have a number of Tridents which originated with Connex, who bought 127 plus operated about 17 from a batch of 22 originally bought for Armchair
Go-Ahead and First used to run Tridents with mostly Plaxton Presdient bodywork (Go-Ahead's fleet was approx 35 until they all left by around 2010) and First and its successors ran about 300 plus (not sure on the exact number). Tower Transit (who bought Westbourne Park and Lea Interchange from First) ran about ten Presidents at Westbourne Park plus hired about ten ex Stagecoach ones from Ensignbus until the 295 was lost to Metroline.
The Plaxton President Tridents are virtually extinct in London, only three left in regular use, all at Sulivans are TPL924/925 (ex Blue Triangle) and PDL26 (ex Go-Ahead London) which retain their original codes from the operators they were new from.
Metroline bought about 250 on ALX400 and Plaxton Presidents with long and short wheelbases, central and forward staircases.
Arriva has never operated the 'traditional' Trident but does operate about 325 Enviro400s which are coded 'T' for Trident.