Used 2 cans for my 18" ACS wheels on my E39. Have 5 coats on each wheel. So 4 cans are plenty for your 20" wheels.

^
WE350Z same pic I post in the reply to your thread on twinturbo.net
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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles)
http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
01 360 Novitec Spider F1 (26k miles)