Just like speeding up crafting time, the spark reduction depends on the power cost of recipes.
For recipes that require no power at all, you’ll get the best bang for your buck with up to 14 advanced coils (I often split the coils from a fully powered machine to a second machine, 12 12, when doing large batches of unpowered crafting), then you get into heavy diminishing returns (in some cases to the point of not even showing in the crafting menu). After 16 advanced coils you’ve hit the cap of reduction for recipes that don’t require power.
The reduction is not a flat rate, its non-linear, and has diminishing returns.
Thus the first coil will offer the greatest benefit.
Let’s say we’re using mass crafts of Refined Titanium Alloy with Advanced Refinery Power Coils as an example:
| Adv. Coils | Time | Spark |
|---|---|---|
| 0 | 50m | 20000 |
| 1 | 38m18s | 15323 |
| 2 | 34m50s | 13938 |
| 3 | 32m1s | 12809 |
| 4 | 29m45s | 11902 |
| 5 | 27m58s | 11187 |
| 6 | 26m35s | 10633 |
| 7 | 25m32s | 10216 |
| 8 | 24m46s | 9909 |
| 9 | 24m13s | 9693 |
| 10 | 23m51s | 9546 |
| 11 | 23m37s | 9452 |
| 12 | 23m29s | 9397 |
| 13 | 23m25s | 9368 |
| 14 | 23m23s | 9355 |
| 15 | 23m22s | 9351 |
| 16 | 23m22s | 9350 |
| 17 | 23m22s | 9350 |
| 18 | 23m22s | 9350 |
| 24 | 23m22s | 9350 |
Edit: And of course, you can only have a maximum of 24 coils hooked up to a single machine.
The first coil offers a reduction to both crafting time and spark of about 23.4%. Second one is a reduction of 9.05% and it goes down from there.