Back in 2013, the PLA already had 16 battalions of ZTZ-99s at 31 vehicles each, and another 40 in training bases. That gives a total of 16 x 31 + 40 = 536 vehicles in 2013.
It's now 2016 and two batches of ZTZ-99A have been produced. The first batch was estimated to be ~100 vehicles to equip 3 battalions of the 112th Mechanised Infantry Division of the 38th GA, and now this second batch of ~60 vehicles will probably be sent to either the 116th Division of the 39th GA or 1st Armoured Brigade of the 65th GA.
So, in total, there are ~150 ZTZ-99As + at least 536 ZTZ-99s = 686 ZTZ-99 vehicles of all variants. This is a minimum count so the actual number is probably higher.