How many CFU do I need?
The best dose is the one that was tested in humans for the outcome you want. ISAPP: "Most probiotics have been tested at levels between 1-10 billion CFU/d", and "A greater number of colony forming units (CFU) does not always equate with enhanced effects." In one dose-ranging trial with over 300 subjects, only the middle dose (10⁸) beat placebo — a result ACG itself calls "confusing."
The cleanest demonstration that CFU is not a dose–response
More CFU is not more effect, and the cleanest demonstration is in ACG's own review. Over 300 subjects were randomised to 10⁶, 10⁸ or 10¹⁰ CFU/mL of B. infantis 35624 or placebo — improvement was reported in the 10⁸ group and not the other two, which ACG describes as "confusing because the lack of dose response was challenging to explain" (PMID 33315591 (opens in a new tab)). ISAPP's own position matches: "A greater number of colony forming units (CFU) does not always equate with enhanced effects… Most probiotics have been tested at levels between 1-10 billion CFU/d."
What the shelf actually offers, counted
Sixteen of the 27 products exceed 10 billion CFU, against ISAPP's own statement that "Most probiotics have been tested at levels between 1-10 billion CFU/d" and that "Simply having lots of strains is not a guarantee of a more beneficial product."
Only 9 of 27 products (33.3%) guarantee CFU through expiry on the label. Another 8 of 27 (29.6%) state no potency basis at all. Five state manufacture-only, two state through-expiry off-label only, one contradicts itself between label and website, and two have no CFU number anywhere.
Why a milligram figure tells you nothing, in NIH's own words: US rules "only require manufacturers to list the total weight of the microorganisms… this cellular mass can consist of both live and dead microorganisms and, therefore, has no relationship with the number of viable microorganisms in the product."