Well, a hard number would take a little research, but as you can see from the 2011 sales, the Z4 is not a volume seller. It's a rare car, and I'm glad for it! It certainly stands out when you drive and prompts lots of head turning.
2006 525 xi, Orient Blue Metallic, Beige leather, Dark Poplar trim, Premium Package, Cold Weather Package
2012 Z4 35i, ED 2/24/12, Melbourne Red Metallic, Black leather, 6 MT, ///
M Sport, Premium Package, Premium Sound Package, Cold Weather Package, Nav, Comfort Access, LED Angel Eyes, BMWPedals pedal covers, stubby antenna, Luxon strut bar, CF engine cover, LCI side marker, powder coated calipers, Extra Enjoyment Package