Distribution of the number of clover leaves
I have found a study [1] of 2017 (http://www.sharetheluck.ch/single-post/How-rare-are-four-leaf-clovers-really) which answers a great part my question, and I'd like to show those results together with some additional evaluation.

They have counted 5.7 million clovers and found the following numbers compared to 3-leaflet clover: 

4-leaflet clover    1 in 5,076
5-leaflet clover    1 in 24,390
6-leaflet clover    1 in 312,500    

For the 4-leaflet clover they have corrected appreciably the common figure of 1 in 10,000 (see e.g. the comment of Untitpoi here).

A logartihmic plot of these relative frequencies shows interesting features

(i) a fairly regular exponential decay of the frequency in the range n=4,5,6
(ii)    a significant "exception" from this rule is the "normal" clover with n=3  



How can these observations be explained?

It would also be interesting to extend the study to n>6, and maybe also to the range of "deficient" clover which posesses only one or two leaflets. The latter are most probably very rare so that the complete log-plot would resemble an inverted letter V.

The question of 2-leaflet clover was raised here [2] https://www.quora.com/What-are-the-chances-of-me-finding-a-two-leafed-clover. But no quantitative answer was given.

References

[1] http://www.sharetheluck.ch/single-post/How-rare-are-four-leaf-clovers-really   

[2] https://www.quora.com/What-are-the-chances-of-me-finding-a-two-leafed-clover

[3] https://en.wikipedia.org/wiki/Four-leaf_clover#cite_note-auto-6
A comprehensive reference which I have only partially explored up to now