It looks like your secondary diameter is about 4.5" in diameter (wound around 4" PVC)?
The more transformers you put in parallel, the more balancing will become an issue. 23mA at 10kV is a bit on the small side for a coil of that size, but that's not to say you wouldn't be able to make it work. More in parallel would be ideal if you can afford it, but take care to keep your run times short, and definitely use all of the recommended filtering (look into Terry filters, if you haven't already) to prevent permanent damage to the OBITs. These transformers aren't really designed to be run in parallel and if there is any imbalance in voltage (which is likely), then they will damage each other over time.