How many words are considered to become subject to verbatim copy?
There is no single "number of words" threshold, because the issue is the degree of creativity and salience in the choice of words, which is dependent on the exact phrase and context.

If those few words are extremely significant and clearly attributable to a particular person, you might need to quote them, e.g.:


  "Eat my shorts" (Simpson, 1997)


On the other hand, some words are just not very interesting, and have a much higher threshold for quotation, e.g.:


  On the basis of the data presented, this study concludes


My recommendation is that rather than stressing about particular thresholds, you read the original source that you are paraphrasing, then set it aside and do something else for a while.  Then come back and write your summary from memory, in your own words; what you produce will almost certainly be different enough from the original to be not problematic.