Accepted answer
There is no count, and 27G is why: coring is a single event, not an accumulation. A needle cores when its bevel punches a full disc of rubber instead of parting it, and that either happens on a given puncture or does not. At 27G the bore is narrow enough that any disc it cuts is small and easy to miss under inspection. Technique moves the odds far more than the count does: enter at an angle, rotate the bevel, then bring it upright — that parts the septum rather than punching it. Inspect against a dark background before every draw, because the number of punctures a stopper survives is a property of that stopper and this is the only way to find it out.
Start with the fact that gauge numbers run backwards — a higher number is a thinner needle — which is the source of half the confusion in this tag.
Typical outer diameters: 21G is about 0.82 mm, 23G about 0.64 mm, 25G about 0.51 mm, 29G about 0.34 mm and 31G about 0.26 mm. The gauge number and the diameter move in opposite directions.
The underlying point is that fixed-needle insulin syringes are supplied in 29G to 31G and cannot be swapped for drawing, which is the trade-off against their much lower dead space.
Needle gauge to outer diameter correspondence is standardised and published; the inverse relationship between gauge number and diameter is the reason for the counter-intuitive labelling.
If a solution will not draw through a 25G needle, the problem is the solution rather than the needle.
Gauge numbers run backwards. Higher number, thinner needle.