Two. Depth perception better with two eyes involved. Cross-dominance (noted by Rick, above) may require closing a non-dominant eye if shooting with an opposite strong-hand side. My son does this with a shotgun: right-handed, but left-eye dominant, so closes the left eye when shooting.
Draw back and focus with two then I close left eye while drawing and release. Same way I shoot a shot gun.(except the drawing and release part) So I guess one, at the time of release. Its just I can concentrate (focus) so much better.
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