Enlarging len which aperture?

The quality of your enlarging lens will lead to better and sharper results at greater magnification. The aperture ( low number = large opening, high number = small opening) you set not only affects the amount of light it lets through but the depth of field as well. A good average aperture is F/5.6 in most cases. If the exposure times are to short a larger F number is needed, this will give you a chance to hold back shadow areas that may become to dark to show any detail. But don't get carried away and close down the lens by too  many F numbers as this may over heat the negative causing it to buckle, making the picture loose sharpness.