The kind that converts
Facetious answer maybe but generally truthful.
It's about ROI -- what can you afford? SEO traffic costs too -- in time and knowledge. A shitload of low quality adult traffic will (may) convert if you can amortize out the cost -- you may need to invest 10K before seeing tangible results.
Because of ad blockers you start out with a 30% -60% handicap -- factor that into the real expense.
PPC (CPC) is the best if the links are not on the block list and are on quality referring websites. SEO organic referrals are very good too.