I believe that recommendation is to use no more than 1/4 of the total amount of juice being from a beet, and the other 3/4 being made up of other vegetables. So, if you are drinking a 16 oz. juice, 1/4 can be beet.
However, I am wondering about where the idea that beets are too potent to juice alone came from. Just because someone said this doesn't make it true. Once upon a time all of Europe thought that tomatoes were poisenous, and they were not eaten. Now only a small minority of humans on the planet consider tomatoes to be poisenous.
I made a carrot/beet juice mixture yesterday, and used two 4 inch diameter beets in it, and as usual did not experience any detrimental effects.