For example, Latin language and Roman numerals. Are people likely to mistake numbers like ID (499) or VIM (994) for words, or is it always clear enough what’s what from context? I think Hebrew, Braille, and maybe Greek also do this, though i’m not as familiar with those scripts as with Latin.
I ask because i’m making a conlang and having a little trouble coming up with enough letters, let alone numerals too. Reusing letters would be helpful and probably not confusing if i make sure that numbers are never pronouncable as letters, but making this easy to read is important to me.
Besides context, like Nemo said, sequences of letters that are legit for numbers are often invalid for common words. For example, in Latin you’ll never see words with “MC” (because */mk/ is phonotactically forbidden), and yet you’ll see it for plenty numbers. Or “III”, it pops up all the time for numbers but almost never for actual words.
On the conlanging part: if you’re using an alphabet, and your phonotactics prevent consonant-only words (pretty common restriction), you can ensure the numbers are obvious as numbers by using only consonants. Another alternative would be to create one grapheme to prefix numbers with; like, instead of writing “ID” you’d write “#ID”.
Most consonant-only syllables are forbidden, so i think i will use the non-syllabic consonants as numerals.