Hi LilaGrace,
There are many more than 6-8 books per 'book band' in ORT. However the vast majority of them are 'non-decodable'. Floppy's phonics and Songbirds are phonics based series within ORT. There are about 6 Floppy's Phonics books per stage and 12 Songbirds books per stage, up to level 6/orange.
There are huge differences to reading abilities in children within one school year. On average, children read levels 1-3 (pink, red, yellow) in reception, then 4-6/7 (blue, green, orange, turquoise) in Y1. Levels 7/8-11 (turquoise, purple, gold, white, lime) in Y2. If they are following a decent phonics programme and use books matching the children's phonics abilities (which they are required to by law, i.e. the statutory parts of the NC; but I'd hazard an estimate that less than half of all schools adhere to this) - then by stage 6/orange books a child should be able to read (as in, decode) 'any' book. From then on books become harder in that they use more complex grammar, longer sentences, the books overall are longer, and they cover less familiar topics. Rather than more phonics.
Some schools are quite good at differentiating and will teach the children phonics at the level they are at, but some will just do whole class teaching and differentiate only in the books given for home reading. In our case that meant that in the whole of reception, the children were taught phonics that DS knew before he started - because, like your child, he had had a strong interest to learn and was like a sponge with what I taught him before school. So it was a re-cap of stuff he knew already. Not a problem really, and phonics 'lessons' were short, not a large enough part of the school day to cause boredom. But the reading books he was given were at the phonics level that was taught in Y1. So I had little choice but to teach him the Y1 phonics when he encountered it in his reading books in reception. Which then meant that phonics lessons in Y1 were just a re-cap again.
Just saying this as one possible problem that can arise from teaching them yourself before school. However I'll point out that some children in DS' year started without knowing any phonics at all, but picked it up so quickly that they flew through the levels and had the exact same issues as DS had - that the phonics teaching pace was too slow for their rapidly growing skills. Others in contrast could not keep up with the phonics teaching pace.
What I'm saying is it's a problem that can arise in any school that doesn't differentiate effectively, and has little to do with you teaching/not teaching your child beforehand.
On Oxford Reading Owl you can take a look at what books in the various bands look like.