With synthetics I would expect one (1) would be sufficient as it is not critical for brush health to dry between uses, but for badger and daily use it would seem at least two (2) are needed if using a small knot and in a relatively dry climate. (My 22mm knot brush takes 24 hours to dry, if not more sometimes.) If using bigger knots and/or in a humid climate I suspect the minimum number to go [way] up. Sound about right?