Before we can tell you how many girds you'll need we need to know:
1) What size cage you are making?
2) Any lofts or second levels?
3) Will you use the grids to make a stand, if so will you be making the stand 1 or 2 girds high?
4) If you're not making a stand or not making one with girds will you need girds under the colorplast?
A 2x5 is the smallest I'd recommended for two boars, but seeing as your two will be going though puberty at around the same time a 3x4 would be what I truly would recommend as It's better to have a big cage so you can lessen the chance of fighting instead of having a 'small' cage then them fighting then having to re-introduce them and make a bigger cage.
And I can honestly say that the bigger the cage the less likely they are to fight as my youngest boar (Ron) who is 7 months old isn't being to much of a pain to my other boars in his cage who are 1 year and 10 months old, Infact there is only the sometimes mounting and no real fighting.