Four — black, cyan, magenta and yellow. Each is sold separately, so you replace only what is empty. On these machines black tends to run out well ahead of the colours, so buying black singles alongside the occasional multipack often works out best.

They fit exactly the same printers — the XL simply holds more ink. If you print often, 408XL works out cheaper per page and means fewer cartridge changes. For light use the standard 408 may be enough. You can run one of each in the same printer without any problem.

Yes. Compatible cartridges are made to the same fit and function as the originals, and for an office printing in volume the saving against genuine Epson is substantial. If the printer is shared, it is worth keeping a spare black on the shelf.

If the printer reports that a cartridge can't be recognised, take it out and wipe the small copper-coloured chip gently with a dry lint-free cloth, then refit it firmly until it clicks. A poor contact on the chip is the usual cause. If it persists, switch the printer off at the mains for a minute before trying again.