You misunderstand 分 here.
分 does not mean "division" here. Here 分 means the "points" (test score), in an exam or a contest where the perfect score is 10.
三分 3 points (out of 10)
五分 5 points (out of 10)
十分 ①[original meaning] 10 points (out of 10), perfect score, full marks ②[extended meaning] "perfectly", very, extremely
幾分 how many points (out of 10)
Assuming that there is a exam called "love", how many points can I get from the exam?
我愛你有幾分？
How many points do I love you?
How deep do I love you?