Quick, easy question here. If you have a Ballistic Mechadendrite, can you fire it along with a weapon in your hand? Or two pistols in your hands? Or what if you have two Ballistic Mechadendrites? Or Eight of them, and two pistols in your hands? Or eight of them and two Autoguns in your hands? (Along with Recoil Gloves, of course.)

1. Can you fire a Ballistic Mechandrite along with your normal weapon?

I'd say yes, since it uses your reaction.

2. With two Pistols in your hand?

Once again yes, because reaction.

3. What is if you have multiple Ballistic Mechandrites?

I'd argue one Ballistic Mechandrite would also "count as an arm", so you can use them like normal weapons, however, you can still only use Ballistic Mechandrites for those reaction to attack thingy. They'd also probably count as off-hands.

4. Eight BMs, Two Pistols?

Same as 3. Firing two weapons from the same hand would be a Multiple Attacks Action and you get one BM-Attack for your reaction. You could also say Multiple Attacks mean "Using all and any means to shoot/stab at the enemy" In which case one might say "Yep, 10 different weapons shooting at the target." It mainly depends on how BMs are handled, as additional off-hands or as, well, mechandrites, or something in between. However, strictly RaW it'd be "Two Pistols, one Mechandrite"

5. Eight BMs, Two Autoguns and Recoil Gloves?

Same as with point 4 I'd say.

Per DH01 Core Rules p.154, Attacking with a Ballistic Mechadendrite is a Reaction and counts as attacking with your off-hand, though without incurring the normal off-hand attack penalty.

Normally you can make a single attack with your off-hand weapon, and since Ballistic Mechadendrites explicitly counts as an off-hand attack, I'd say this covers Ballistic Mechadendrites too.