Size of beach garrisons:
If I recall correctly, there are 12 or 13 beaches for Germany to guard (from Denmark to southwestern France). Having a large number of divs on each beach costs a huge amount of IC and MP. Usually the beaches are left to second-class units from Balkan minors, and nowhere near 24 divs per beach (usually 3-5 divs per beach).
6 divisions too little?:
Attacking and winning on a above mentioned beach should be possible with 6 divisions. Sure, it might not be very easy with 6 vanilla inf, but this is the whole point. Invasions should require better units (marines, tanks, some nice brigades), air support (no more landing 24 divs in Hamburg without air cover), use of paratroopers (have you ever seen people use paras in HoI like they were used in WWII?) and some BB support (no more 24 TP escorted by only subs).
Concerning how to limit the invasion force:
We used the TP/10 rule in one of our DEG games (during 1.05c). It did work rather well, most invasions were only a few divisions (2-3). But the system we used in the latest 39'ers game (max = 6) was a lot better, since it is simple to use and simple to remember. I guess you could combine the two systems (like TP/10, with absolute cap at 6, or something similar), but this only complicates things. I am in favor of simple house rules, and the max 6 rule is just that: simple.
Remember that the rule works both ways. Max 6 divs does make it harder for UK/USA to make a D-Day landing, but it also makes it harder for Germany to pull off Seelöwe.