Initial Posting: 4/9/2016    Last Update: 4/13/2018
Answer: As a general rule, a probiotic should provide at least 1 billion CFUs (colony forming units, i.e., viable cells), with doses typically ranging between 1 billion and 10 billion CFUs daily for adults. This is explained in the Probiotic Supplements Review, which includes ConsumerLab.com's tests of popular probiotics. There are some exceptions to this: Doses as high as 100 billion CFUs or more are used in some situations, while doses for children and infants are sometimes below 1 billion.
The specific dose will depend on the probiotic strain and the purpose for which it is taken. You can look this up in the "What to Consider When Using" section of the Review, where you will find the dose and strain(s) used in preventing/treating antibiotic-associated diarrhea, IBS, anxiety, depression, allergies, and more. Be aware that probiotic supplements may contain a single type of organism or a combination of many types. Combinations have been more effective in some situations, but single strain products have also been effective, so it is not clear that one is better than the other. Details about the clinical studies are found in the "What It Does" section of the Review.