That is the theory. But in real life off grid practice it is not so critical. People do run parallel connected batteries all the time, even though technically it is not best practice. We run a triple parallel bank of golf cart batteries off grid. Three series strings of four 6 volt GC-2's in parallel. They have been in service for 5 years 7 months and still serve us well. They are monitored, fluid levels kept up, not discharged more than 1/3, usually less, never left in a state of partial charge more than 24 hours as a rule and equalized when the cell sp. gr. differences get to be greater than 0.015. Ocassionally we selarate the parallel strings and equalize each independently.
If possible it is always best to use series only, no parallel batteries. But in the real world that sometimes means batteries that are more expensive than what we want to spend, to have the amp hour capacity we want. So we make compromises. With good monitoring, sensible use and maintenance it can work outwell. Experience has shown that three parallel strings should be the limit. We also use cables much larger thannecessary to reduce resistance and connect the series strings to heavy copper buss bars.
I fully expect to get 7 years or more out of the 12 golf cart batteries. Not too bad for a technically inferior battery bank.