Rated 6/10
The blueprint before the building was finished — Paul Di'Anno's rawer, punk-inflected vocal approach giving this debut a grit that the more polished Dickinson era traded for operatic ambition, which is either a feature or a limitation depending on what you're after. The galloping bass lines and twin guitar attack are already fully formed, the compositional ambition audible in the multi-part title track, and the NWOBHM energy is genuine throughout.
The honest limitation is that "Hallowed Be Thy Name" doesn't exist yet, and that song was doing significant heavy lifting for the four that Number of the Beast earned. Good moments are real and scattered across the runtime without accumulating into something that demands the higher rating. A respectful three for a band finding what it would become — interesting as a historical document and less essential than what followed.