According to the list of Cardinal Electors here, there are 45 Cardinals aged between 75 and 80, sixteen of whom are listed as "Emeritus", and can therefore be presumed to have retired. Of the remaining 29, nine have curial appointments.
That means that there are 20 major Sees whose Bishops will have presented their resignation to the Pope.
There are nine 74 year olds: four in Rome, five in Metropolitan Sees.
Two reflections: we shouldn't worry too much about the delay in naming the successor to Westminster: he's well down the queue. And we might think that Pope Benedict has the opportunity to shape the Church for the next 10 or 20 years if he gets this right.