Hi Steve: 

Yes there is a theoretical amount and according to one of the ON4UN's low band dxing additions, its 64 at a quarter wave length of the lowest band. . This would make them 66 feet for the 75M band.  
Now my Dad gave me a Chunk of is 2 acher vegetable growing field to build on so of course, this had to be taken into account when placing my antenna. My Dad's vision isn't the greatest but he feels he can see well enough to drive regularly and run farm equipment, mainly an ATV towing a plou and such and he's not about to care if he runs over a wire or 2. LOL 
So I had to pick a corner of my 160/130 lot where I knew he'd never go, even if he was coming across the back board of the property for a turn around and such. 
So the spot can only really accommodate 25 25 foot radials, perhaps a few more but I'm thinking that I'd have to double up to make a significant improvement. I may try another 7 though just to experiment sometime this fall. 

73: 
Mike VO1AX