Calling them letters is a little misleading. There’s 6 dots per character, which gets you a total of 64 possible characters, including spaces. 250 is probably counting all the multi-character contractions and abbreviations.

These cheat sheets do a really good job of condensing the whole system into one page:

https://www.pathstoliteracy.org/resource/braille-charts-summ...

It is surprising!

Unified English Braille which has replaced the older English Braille American Edition uses a lot of "contractions" ('ea', 'be', etc.), shortform words which are combinations of braille (like the braille for 'ab' which can mean 'about'), and wordsigns ('k' for 'knowledge', etc.) in the Grade II forms.

Grade I Braille is closer to what you thinking of.

It's kind of like when you first start studying American Sign Language and realize that a lot of the grammatical structure comes from French Sign Language.

It looks like "250 letters" would be better described as something like "250 characters." Since it goes on to show patterns for numbers, punctuation, short-hand for some words, common prefixes and suffixes, etc. I wouldn't consider these to be "letters" in the English alphabet.

The reason is that Braille can't really be resized and still be readable, so letter cells are fairly large. A normal letter / A4 sized paper fits maybe 28 columns of text, so Braille is often embossed on legal size paper. And Braille pages don't lay flat against each other, so the books end up being enormous. The paper itself is also thicker because it holds dots better, so the books are quite heavy. This is why so many contractions are used in printing Braille.

Some languages use few or no contractions in Braille, but I think many of them also have very few Braille books available.