Generally, as long as 2% transistors are failed in a CPU, the CPU will be unusable.
As we all know, high integration CPU which contains plenty of transistors.
Let’s take Intel as a example, the high-end core 6 has around 2,000,000,000 transistors, and the I7, I5 series has 1,000,000,000 ones, even the low-end Celeron G1610 contains 7,000,000,000 transistors.