I've gotten up to about 75 miles a week in singles. Imuch prefer running doubles, though, so I haven't tested out my max for this.
A friend of mine, however, does all his running in singles, as this fits better with his schedule. He says he can get up to 88-92 per week, consistently, in singles. Any more than that and it starts to get pretty tough.
The thing to remember is - and this applies to a lot of situations - your weekly mileage is just one variable in your training. 75 miles in singles is not the same as 75 in doubles, just as 75 miles with three fast, hard workouts is very different from 75 miles of easy running. Mileage is an important variable to consider, but it is a very incomplete description of your training; don't put too much weight on that one, single number. If your mileage is all in singles, other people who are running a lot of doubles may well be putting in more miles than you, but that doesn't necessarily mean they're doing better training; apples and oranges.