As it is you can grow more than 15 plants for personal medical purposes. See below WAC 246-75-010 section 3a
(1) Purpose. The purpose of this section is to define the amount of marijuana a qualifying patient could reasonably expect to need over a sixty-day period for their personal medical use. It is intended to:
(a) Allow medical practitioners to exercise their best professional judgment in the delivery of medical treatment;
(b) Allow designated providers to assist patients in the manner provided in chapter 69.51A RCW; and
(c) Provide clarification to patients, law enforcement and others in the use of medical marijuana.
(a) "Designated provider" means a person as defined in RCW 69.51A.010.
(b) "Plant" means any marijuana plant in any stage of growth.
(c) "Qualifying patient" means a person as defined in RCW 69.51A.010.
(d) "Useable marijuana" means the dried leaves and flowers of the Cannabis plant family Moraceae. Useable marijuana excludes stems, stalks, seeds and roots.
(3) Presumptive sixty-day supply.
(a) A qualifying patient and a designated provider may possess a total of no more than twenty-four ounces of useable marijuana, and no more than fifteen plants.
(b) Amounts listed in (a) of this subsection are total amounts of marijuana between both a qualifying patient and a designated provider.
(c) The presumption in this section may be overcome with evidence of a qualifying patient's necessary medical use.