> Mathematically, for SHA-1’s 160-bit output, the birthday bound means that you would need about 1.4 septillion random files (1.4 quadrillion billion files - 1,400,000,000,000,000 billion - it's impossible to effectively describe) in a single project to have file hashes accidentally collide.

The birthday problem gives the number of hashes to have more than 50% probability of having a collision. To have a collision, only 2 hashes are needed, with very low probability.