Well, probably not too much haha! If it's a strong flavor, then perhaps a few mLs would be ok. I can tell you that, for example, I got a bottle of MBV Sour Yum with 3 extra flavor shots, at 36mg, and I can mix 20% this
juice with 80% VG and it's a pretty decent vape and only about 7mg nicotine.
Just keep in mind that you are going to dilute the flavor as well as reduce the nicotine amount per mL. You could add 5 mL of VG and you'd only get the
juice down to about a 20.5 mg/mL. Adding 15mL of VG would get you to a 12 mg/mL, but would really reduce your flavor.
Also consider that you are going to be thickening your liquid, if that is relevant to you. Hope I was at least a little help!