If you're looking to find the voltage rating, it depends on the configuration. I'll give you an example:

Let's say you have a 1000 uF capacitor with a voltage rating of 16v. If you put two of these capacitors in series, you'd cut your effective capacitance in half, to 500 uF, but since only half of the overall voltage is dropped across each capacitor, the effective voltage rating of the pair would be doubled to 32v.

Now, let's say that instead of putting them in series, you put them in parallel instead. In this case, the voltage rating would remain unchanged because the same voltage would be dropped across both caps. However, the effective capacitance would be doubled to 2000 uF.

Now let's say you form a small bank of capacitors by putting two 1000 uF caps in series, along with a second identical series pair in parallel with the first two (for a grand total of four capacitors, two in series and two in parallel). This would result in an effective capacitance of 1000 uF for the bank, with a voltage rating of 32v.

So for an arbitrary bank of series combinations and parallel combinations, you'd end up with the following properties:


,

where and are the capacitance and breakdown voltage rating of a single capacitor out of the bank.

Woops, that last line should say BV_0, not VB_0.

Calculating may be approximate. The best method is by measurements. Connect a datalogger or power analyser to the main CB and record parameters like V,I,pf,Hz,Var,W,VA etc over a period of 24hrs. From this data you know the maximum and minimum pf,I and Var.Then you should study the pattern of loading(motor hp vs time)and sizes of load steps. If all loads are small you can use smaller capacitor units and if large motors are used
Larger capacitor units too should be used. You should decide how many and sizes capacitor units are required. The best advise is give the data to a capacitor bank manufacturer and ask for a quotation.