You can go a lot more than that, the Super Yoghurt approach gets you to about 250 billion for 100ml or so.
I am actually now going to attempt the the super yogurt recipe (involving inulin) you posted above.
One question I have: if this recipe can produce 250 billion friendly bacteria per 100 ml, so that would be 2500 billion in a litre, then just 1 litre could last you a year, if you took a dose of about 7 billion bacteria a day. So that would be amazingly economical.
But I wonder how you might preserve the 1 litre containing 2500 billion bacteria. Would you have any ideas about that?
Do you know roughly what the shelf life is of this super yogurt, if kept in the fridge?
If you made up a whole litre, perhaps you could divide the 1 litre into say 12 equal parts (one part for each month of the year), and freeze them.
Then each month, you could take one of these 12th parts out from the freezer, and consume a little bit each day, keeping it in the fridge. That might work, provided the 12th part would last for 1 month in a fridge.
Or I guess another approach might just be to make a batch of this super yogurt every month, leaving it in the fridge, and consuming a little bit each day. I just wonder how long it lasts in the fridge.