Wrong section, however some quick math:
25kva at 120V = 208.333A ( Single Phase)
A 1k HPS at 120V = 8.3333333333A
So Transformer at 208.333A / 8.3333333333 = 24.999 Lights ( 25 Lights due to % fluctuation)
So theoretically you can fit 25 Lights on a 25kva Single Phase Transformer at maximum output.
Building a bigass grow?
A electronics and power forum would have been a better place to ask more serious electric questions .
Hope that helps.