In Special Chapter, you should generally hit stage 4 or higher if possible for the best chances of getting biometrics.
When you get 100% of the given stage, you can unlock “no limit” which you can play the specific stage for unlimited times within a 30 minute period.
There is a chance that you will not get any biometrics at all for the level, however that chance seems to be less as you fight higher level stages.
Stage 1~3 will not drop more than 1 biometrics.
Stage 4~5 have chances of dropping 2~3 biometrics.
Ant-Man Special Chapter
At the time of writing this, the shifter stages have 100% chance at spawning the shifter. However, you do not get biometrics 100% of the time.