Following PROPER battery charging procedures which calls for a discharged battery that has 20% charge remaining or 80% used) you follw the 8 8 8 rule - 8 hours charge time, 8 hours cooling time (that allows full battery voltage to be restored) and 8 hours run time you will need 12 batteries. But also, most trucks do not accumulate a full 8 hours of actual running time (hour meter hours) in an 8 hour shift. The most actual running hours I have seen in an 8 hour hift is 6.5 actual operating hours (the average run time is about 3.5 to 4.5 hours and less on the second & third shifts). You will need to determine this by the hours your current fleet is accumulating.
Picking the RIGHT battery for your needs in you application is best done by using a test devise called a POWER PROVER (at least in the US that is what it is called) that is connected on an electric truck that you are considering. The POWER PROVER will tell you how many battery amps are used per work shift. With the run time hours per shift you can determine the amp/hour needs and you can determine the amp/hour capacity of the battery you need to run 1 shift or 1.5 shifts or 2 shifts. As a minumum you need 2 batteries per truck. Now you will need to get a rental/demo unit with the same voltage & as close to the specifications including any hydraulically operated attachments you are you are considering from your lift truck dealer & a POWER PROVER from the battery supplier (the battery supplier may have a different name for the same thing). The battery sizing should not be compromized by lower priced alternatives (i.e. lower amp/hour capacity).
If you are considering electrics, which can save you $$ (lower fuel cost, less maiantence, clear running, quieter, etc. etc.) in the long run IF they are sized right for your application and correct battery/charger maintenance is utilized - make a mistake & they will be a pain & cost you money. Do it right the first time.
Without knowing more information & seeing your operations, work environment, floor condtions, fork lift job operations etc. etc is very difficult , actually impossible to answer you questions. Changing to electric cannot be done by paper study only - try (rent) before you buy.