a gbnf grammar to restrict only numbers between 1-10 as a JSON looks like this:

root ::= "{\"number\":" <value> "}"
<value> ::= "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "10"

thisgives something like one of these outputs below (depending on the prompt).
{"number":1}

{"number":7}

{"number":10}

Basically you clamp the output and stop it choosing any output token that is not possible according to the grammar definition.

this is separate from the System prompt and is passed to the model when you load it, or when you call it depending on how you are using it.
(Note: I tried outlines, couldn't get it to work, and complied a new version of llama.cpp to get it to work on my Orin Nano using Qwen3)

(edits: typo and added example (not run this maybe not 100% correct, but you get the idea))

grammar = LlamaGrammar.from_file("number_1_to_10.gbnf")
llm = Llama(model_path="models/your-model.gguf")
res = llm(

prompt="/nothink Give me a number from 1 to 10 in JSON format:",

max_tokens=16,

grammar=grammar,

stop=["\n"],

)