(this is referring the 'at most one image from each [...]' option in the 'Advanced Search' function)
The software that drives our search can only at this time return one image from a particular 'group', such as grid square or month. So at this time, can't select multiple images per group.
Also note there is no control over which image is selected, at the moment it's pretty much chosen at random.
Can use the 'Image Browser' (just called 'Browser' in the links on the side) - in the 'Grouped' Mode, can now select multiple images per group. E.g. can get it to show say 4 images per square.
That has much more controls for configuring the sampling.