Every power backup system should be implemented to stand the test of time. This is because the cost of investment on them is quite high. Good batteries are expensive and it will be a wastage of financial resources to install them in such a way that they won't last.
Depending of the capacity of the inverter, the size of backup batteries and the backup time, single battery or multiple batteries connected together can be used.
What makes up a battery string or bank?
Advantages of battery strings configuration
To add or remove capacity as needed
Advantages of using a single battery instead of battery string