"Most bacteria have at least one cell wall"
I agree with @Maximilian that the phrase you quote is unclear, and we can't understand if it's talking about the number of cell walls of about the presence of cell walls:


  Most bacteria have at least 1 cell wall, meaning that some bacteria can have more than 1


This is wrong, as @Maximilian explained. But this:


  Most bacteria have a cell wall, meaning that some bacteria doesn't have any


Is correct. Tenericutes, a group of Eubacteria, don't have cell wall:

http://onlinelibrary.wiley.com/doi/10.1002/9781118960608.pbm00025/abstract

Reading the answer, it's clear that the user was talking about the outer membrane in Gracilicutes. 

However, it's good to emphasize that all bacteria have only 1 cell membrane, because the outer membrane is not defined as a "cell membrane" (despite being a phospholipid bilayer): http://www.nature.com/nrmicro/journal/v13/n10/images/nrmicro3480-f1.jpg