The detail
There is no fixed token count for a sentence because sentence length varies widely, but English sentences average somewhere around 15–20 words. At the standard ratio of about 1.33 tokens per word, that puts a typical sentence near 20–27 tokens.
Short, punchy sentences can be under 10 tokens; long, clause-heavy sentences with technical terms can exceed 50. Punctuation, quotation marks, and numbers each consume tokens too, which is easy to forget when estimating from word count alone.
If you are sizing prompts made of many short instructions, count the whole block at once rather than multiplying a per-sentence average — tokenizers merge spaces and punctuation across boundaries, so the total is usually a little lower than the sum of parts.