Off topic because SP only, but ...
It depends on sacred units, national priests level, and factors like possiblity of expanding and money availability (to build temples), but usually, I take 5-6.
Sometimes more, and in these cases, I go 9-10 for sacred blessed strategies, or to really benefit from my chosen scales if I found it strategic.
Sometimes again, if I can really count on my priests and my money, I go lower, but I never tried lower than 4 ... except perhaps in my really early days of Dom2
I usually don't care about dominion until I have securised a base of provinces and have money running to build priests and temples. Then, at this moment, I attack this part and build maximum temples I can afford to rise my domminion at maximum level.
Two reasons (not exclusive): rise maximum recrutability of sacred, and/or benefit from my scales (say heat for Abysia or cold blooded nations, cold for Niefel, and so).
Of course, if I am too low in dominion and near a possible domkill, I react before.