Until the standard cars are eliminated.
Then we'll begin to see where PD struggles: efficiency in car modeling. Maybe they've expanded staff (or outsourced), but the DLC is not demonstrative of that just yet.
GT5 Prologue launched with 37 cars in Dec. 2007; the final count was 76 by Apr. 2008
GT5 had 220 ish in Nov. 2010
Since then, there have been 3 karts, 1 X-Wing*, 1 Schulze GT-R, 11 TCs, 10 cars in two car packs, 1 Toyota GT86, 1 Toyota FT-86 II, 1 Scion FR-S... probably others I've missed. (EDIT: 11 NASCAR cars, 1 extra Kart - 41 cars, 2.2 per month)
So, in the 19 months since release are we to believe they've only modeled 29 cars, of which several are practically only amendments of existing cars? That's only 1.5 cars per month.
GT4 was released in Dec. 2004, which puts 220 cars in Nov. 2010 at 3 cars per month.
3 cars per month equates to only 60 cars produced since launch, which means (at 6 months per car) there are only 18 car modelers.
But cars took 60% of the development time, which is 60% of the manpower, which is effectively 60% of the staff, which is about
66 people. And 6 months per car is an exaggeration as an average figure.
Plus, it's likely that they started much later than Dec. 2004 (not least because of the US and EU releases of GT4, plus TT, plus GTPSP), and also that the first few cars would have taken substantially longer to produce than they can do them now.
So the real figure (today) has to be closer to 10 cars per month (sound familiar?)
In short, the DLC is only "demonstrative" of the DLC, not PD's total work output, which is impossible to gauge at any point because we have no idea about what they're working on.
* Credit to PAPPACLART for this, my new favourite term for the RB X-cars.