You could probably do it mathematically, but I can’t remember the formula for the length of an ellipse’s perimeter given its axis lengths.
But if you are happy enough with an approximate answer, let SU do the maths.
Draw a circle 12" R, and perhaps with more than the default 24 sides - say 48 or 96. Align the vertices with the red axis [EDIT] Or perhaps better, the midpoint of an edge of the arc with the red axis.
Use the Scale too to expand the major axis to something bigger - say 16", as you know you need it bigger than the 14" you started with.
Draw the major axis, and delete the ‘back’ half of the resulting ellipse.
Highlight the edge of the half-ellipse.
Let Entity Info tell you the arc length. Divide that by the number of openings you want. See if the spacing is reasonable. If not, adjust again using the scale tool until you get a workable compromise.