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Amdahl's law caps speedup at 1/(1-p) no matter how many processors are added: with 85% parallel code the ceiling is 6.7x. Adding a per-core overhead term shows why speedup often peaks and then declines as cores are added.
Amdahl's law
S = 1 / ((1 - p) + p/N); ceiling = 1 / (1 - p)
The serial fraction becomes the whole runtime. At 85% parallel, 16 cores already achieve 5.9 of the 6.7 maximum, so further cores are nearly worthless.
No. Gustafson's law argues problem size grows with machine size, which gives a far more optimistic scaling picture for many real workloads.