Solenoid : how dimensions ratio affects the magnetic field?
For a single turn you can use this relationship derived from the Biot Savart law: -

There is also a handy calculator that does the number crunching for you too. So, you get a single number for the flux density at some distance along the Z axis produced by current in the loop.
Then, if you have two current loops then the diameter has to halve and also the 2nd loop is a little further away from the &quot;measurement point&quot;. Calculate two flux densities and add them together.
Rinse and repeat for the number of loops (turns) that you see as appropriate and plot a graph. Using a spreadsheet seems sensible.
At a certain distance away from the loop, I'll bet a single loop will be best but, that's just gut feeling. Close up to the loop, multiple loops will be best.