To wrap just the sides, do some math:
#1 Measure the circumference of all pans required for this cake (using a sewing tape measure) adding up the measurement of each pan
#2 Measure the circumference of a cake pan for which you have already made such a cake and therefore you know how much modelling chocolate you used. Call this amount "one batch".
Divide the big total #1 by #2 and that tells you how many "batches" of modelling chocolate you need. Add 20% for insurance.