in a multi-shift operation the ideal application is one battery per shift.
this way you would have one battery in the truck
one battery charging
one battery in 'cool down' stage (which should be ready to use when other battery is charging)

it will be a constant rotation

having a battery delivery system in place will help make changing the batteries out alot easier.
This system would consist of a battery station with capability to move the batteries in and out of a roller tray at the station and on and off the lift truck which would have a battery removal tray option in it's battery compartment.
There is also the option to have an overhead lifting device such as a hoist on a I beam that can traverse across the battery station to a loading/unloading zone where the battery can be changed out.
I have also seen attachments that can be mounted to a palletjack that will allow battery removal and installation.

There are different types of battery handling systems out on the market that can accommodate what you would need.

As for what voltage battery to use? The most common is 36V or 48V with the 36v being more common. (in some cases depending on that type and model lift you get it may be 24V.

72v systems are not common any more and batteries would have to be special ordered in most cases.
In some new trucks like linde and jungheinrich i have seen 80v systems coming out for the past couple years. Most of these 80v trucks we sold a hydrogen cell battery pack was bought and installed on these trucks but i'm sure regular batteries (lead acid) or sealed maintenance free type batteries can be obtained as well.
In either case for the above mentioned they will require regular maintenance and for a hydrogen cell system you will have to depend on a licensed trained tech specifically to maintain those.

In my opinion as a seasoned lifttruck tech for the past 30yrs i would say that the lead acid type battery would be the most practical and cost effective way to go.

the sealed maintenance free batteries could also be an option but they do cost more and do require a special charger but do not require hardly any maintenance.

The hydrogen cell battery system is environmentally friendly but is very expensive and maintenance is also very costly.