*I am not a Medical Professional*

Human DNA has over 3,000,000,000 base pairs.
The PCR test makes a replica of that 3,000,000,000 base pairs every cycle.
Cycles - DNAs
13 4,096.00
14 8,192.00
15 16,384.00
16 32,768.00
17 65,536.00
18 131,072.00
19 262,144.00
20 524,288.00
21 1,048,576.00
22 2,097,152.00
23 4,194,304.00
24 8,388,608.00
25 16,777,216.00
26 33,554,432.00
27 67,108,864.00
28 134,217,728.00
29 268,435,456.00
30 536,870,912.00
31 1,073,741,824.00
32 2,147,483,648.00
33 4,294,967,296.00
34 8,589,934,592.00
35 17,179,869,184.00
36 34,359,738,368.00
37 68,719,476,736.00
38 137,438,953,472.00
*Note: at higher cycles, the amount of new DNA generated slows down from 2x the previous cycle, to eventually add no new DNA, because the excess material for replicating DNA in the cells (which the DNA came out of), has ran out.

The amount of Cycles you run is determined by timed runs.
The PCR tester cannot visually see the DNA multiplying, therefore, they use a chart and temperature to estimate the number of cyles that have actually occured.

Denaturation
Temp: 95°C. Time: 5 min on initial cycle; 30 seconds to 1 min on rest

Annealing
Temp: 5°C below Tm of primers; no lower than 40°C. Time: 30-45 seconds. This is the step where you would use a gradient.

Extension
Temp: 72°C. Time: ~1 min/kb of expected product; 5-10 min on last cycle.

Number of Cycles
~30 cycles

2012 video on PCR - youtu.be/4sXPUbIrh3A?t=44

Every cycle, there is a different chance for DNA mutation, which occurs at higher levels for different DNA samples.
If DNA in the body mutates, the cell it is in, is known as cancerous.
Different people with different DNA have different risks for DNA mutation.
Therefore, PCR accuracy is an aggregate measure of many different DNA samples.