If that one person is just making appointments and letting floor salespeople sell and deliver the car, then I would say a good Internet rep could handle 300 per month.
Then they wouldn't be an Internet Sales Manager, but a BDC.
If the "internet person" is in reality a BDC and they aren't doing any actual sales, the number would go up but I still think 300 leads for one person is too many. If you want to effectively work the lead, whether as an ISM or a BDC, and follow up appropriately, it's not about how many phone calls can a person make in an hour. That's what I call "burning ups". It's quality, not quantity, that will increase your closing ratios.
Reality is though, in MOST dealerships, the internet department has salespeople that work the leads AND sell the cars.
In that case, my statement above is my opinion on what they could handle.
To add to that statement, the HIGHER volume that internet department is doing (ie. the ISMs are too busy selling cars and don't have enough time to properly work the leads), the number of leads each person can handle decreases. At this point, you either reduce the leads or get more personnel and you can't just stick anyone in an internet department and expect performance. An effective ISM has a certain skill set that a great floor salesperson may not. I've seen many "eagles" turn into chickens when going from the floor to the internet department.
I've been an Internet Director for 2 different multiple rooftop dealer groups. One 9RT group which contains within the group the #1 Honda dealer in the world that sells about 1000 units/month (in the one store) and has 15 internet managers in the one store. I've also been over a dealer group with 6 RTs with a 30 person BDC and 14 internet managers. I think I have a good feeling for what a good internet manager (and a good BDC employee) can handle.