In the PHB, they have "sample alphabets" for a few of the exotic languages (everything that isn't "common"). For example - the Draconic alphabet:
They suggest these are "sample" alphabets only, so do they have characters for digits in these alphabets as well?
Side note: to clarify, I am perceiving the use of the word "sample" as only part of the character set, not as a suggested character set. If this is incorrect please do inform me.
In general, the other languages within the Forgotten Realms do have numerals. For example, the Draconic Alphabet with numerals can be found on the Forgotten Realms Wikia.
Though I could not find written Primordial or Supernal alphabets, given that the other written instances of alphabets have numerals, I think you are safe to say they all do.
However, I think you are wrong in your interpretation. I do not think they are saying that this is part of the Draconic Alphabet, but rather a possible Draconic Alphabet (in particular from the Forgotten Realms), and neglected to include the numerals due to space constraints.
All non-mechanical information can be treated as suggestions. And some of the mechanical stuff, too, if you wish. If your dragon races need digits, they can have digits, or they can spell out all their numbers. It's your game, you get to own it.
A realistic solution for no numbers is to do something like the Braille system and reuse the letters from A-J as numbers, and then use Arabic notation, not Roman. I believe Hebrew also did something like this. You'd add a notation to indicate "this is a number". Braille basically uses # in front of all its numbers, although currency signs and a couple of other things are considered "obvious". So $EAB.IE is $512.95.
What really bothers me is the omission of rather more fundamental stuff, like punctuation, mathematical operators, how the text is emphasized, or instructions on how things are even written (right to left, up to down, in any random direction that looks pretty like with Ancient Egyptian).
But then you fall down the con-lang rabbit hole, and for that it's easier to just say your Orcs speak Klingon, the Elves speak Tolkien's Elvish, and Dwarves speak Esperanto. Or German, Mandarin, and Swahili.
If you don't get a better answer, you could use Roman numeral style writing of numbers. Dedicated number characters are a fairly recent addition to the Latin character set, having been borrowed from Arabic within the last 1000 years (give or take a few hundred).
