I found an online manual for your UPS unit. The manual specifications show that the six batteries are connected in series. This means that the system voltage requirement is a nominal 72vdc.
You would not be able to reduce the number of batteries unless you have access to a 72vdc battery pack.
The other important piece of data from the manual specifies a maximum of 216 ampere-hour capacity. This means that the six batteries you select should have a capacity of no greater than 216 ampere-hours each. If you do not use such a super-large capacity battery, see below.
The diagram for this device also shows the circuit being able to accept parallel-series strings. If you have, for example, twelve batteries, connect them in strings of six, then parallel the two strings.
Keep all batteries in one string with the same ampere-hour capacity. You could, for example, have a string of 6 batteries, 12vdc, 10 ah and another string of 6 with 20 ah capacity. That would give you 30 ah capacity, well below the maximum of 216 ah noted in the manual.
The result of this type of connection will be increased back-up time.