I'm not sure what the effects there are for it being specifically for frequency regulation. However if you want the batteries to be giving out 10MW at 33KV you will need to have an inverter that can supply 33KV (which is alot!) But if you managed to get it you would need to have 3300 amp hours or 10MW hours! (for one hours of supply).
With just the 480V inverter to work out how many extra modules you would need (assuming you are trying to supply 10MW with it and are ok with it being at 480v not 33kv, because adding extra modules wont increase the amount of volts its giving out, it will only increase the amount of power/current available for its load)
Then you take the amount of power you need to be supplying, e.g. 10MW and you take off the 588.2 Kw (KVA = KW) to get 9.4MW, then dividing that by 8.5w you end up needing to get about 1200 extra modules. Its gonna be a similar order of magnitude for the batteries and essentially the same way to calculate it - however there is the added element of Watt hours, which is how much charge the batteries can give out over an hour. presuming that 10kw is the maximum the batteries can give out at any time (but they can do so for 2 hours, divide the Kw per hour- 20kw by the amount of power being given out, e.g. 10kw to get 2 hours). then its the same proccess of that 10MW taking off 500kw to get 9.5MW and then dividing that by the size of the extra modules (10) to get about a thousand extra modules.
Hope this helps, lemme know if Ive misunderstood the question - I cant add comments as I dont have enough rep