Hi, I'm a homeopath.
Arnica is not indicated for preventing tearing (it is never indicated when the skin is broken - so also not a good choice for C-Section healing at all!), so TBH I'm not sure why you would take it at all?
If you were to take it (for the following reasons: exhaustion, emotional trauma, inability to urinate, feeling sore, lame and bruised, long labour).
Anyway, taking it prophylactically (before labour) is pointless, if you do want to take it the best way is to take one pillule only dissolved in water and sip throughout labour (rather like some people take rescue remedy), then carry on sipping after labour until you feel an improvement. You don't take lots of pills every day, that's not homeopathic dosing - just modern day marketing to sell more!
The potency will depend on what it's being used for, purely physical 30c, purely emotional (emotional shock etc..) 1M, halfway house 200c
HTH!