Hey Bryan, I believe it's 32. After that 2 things may happen:
You don't have enough current to drive more Neopixels (Neopixels use a lot of current)
or you run out of memory (The sketch takes up like 94% of memory and each neopixel has a chunk of memory it uses.)
And actually, doing the math, with only 1.5 amps, the circuit can only drive 25 pixels at full brightness in the color white. I guess the capacitor helps with this since the shift light will never be a constant white color.