There are numerous probiotic products out there. There are the one strain acidophillus in "freeze dried" powder or tablet form. There is also the liquid yoghurt or other liquid form that may have 2-4 strains. Then there is just one 100% live active bacteria with 12 stains. These can go to work immediately and effects felt quickly. Of course, whichever you choose, you must also make sure they are protected when consumed that they are not destroyed by the strong stomach acids. Bacteia count, though important, need not be in the billions. My view is that if they are good quality, anything from 50 to 100 million will suffice.
The reason you need multi-strains because different strains work at different regions of the intestines - large n small. Then there are colonizing (permanent) which is important and transient which is equally important. The colonizing prevents harmful and pathogenic bacteria from coming back to create problems and allow recurrence.
What you choose will depend on how important you think your health and its maintenance is. Obvlously, the better quality would be most expensive but more effective and perhaps in the long run cheaper??