The are 50-odd schools in the borough. That's completely different to the catchment area.
The catchment area of a school is just the region bounded by a circle that's big enough to contain enough kids to fill their reception class.
In many cases, people will live outside of school catchment areas, or will live within a number of schools' catchment areas.
For numerous reasons, the closest school might not be the most appropriate one to attend (ease of travel, sibling attendance, moving house etc).
This method (listing six choices, with various criteria for acceptance) was brought in because you ended up with the situation where parents could effectively 'buy' their kids into a certain school by purchasing a house that was guaranteed to fall within the catchment area, and you ended up with certain schools having tiny catchment areas, and other, less desirable ones, with large catchment areas. Scotland still has the latter situation..