doesn't matter, it could be as high as 50% if someone was quoting chunks for a particular piece of work, but whomever is marking it will see you've referenced it and it won't be a big deal, I generally had at least 15% purely from the in-text citations themselves, then the reference list at the end, but obviously it's all mostly 2%,4% etc. Even a string of words like "the higher concentration of Ethanol in the sample of" can be flagged up, when it's clearly just a broad statement used thousands of times, if you haven't plagiarized, ignore turnitin.