3 for the rifle and five spare in a leather cartridge holder which I bought from Cottis - that's what I take locally anyway, One or two Roe, Muntjac or Chinese Water deer is usually enough, and it leaves me spare rounds just in case of follow up or zero check
On a trip to Scotland, or when going for driven boar, I'll pack as many as I can into a lockable cash box, that's at least 60 x 30-06 (on the basis of better to be looking at it than to be looking for it), but I would only carry 10 or 15 on any stalk/drive unless I had been warned that it was going to be a slaughtering match. I'd leave spares in any vehicle, but I don't tend to do that when stalking locally.
On a fallow cull, I'll carry at least 20 spare in plastic 10 round strips, with 4 in the rifle - just in case - I carried a dozen on one occasion and ran out of ammo, I don't want to do that again