24 Plants can be grown for certain medical conditions.
Lucky you, you can here, if you searched the web, it might have taken a good chunk of your afternoon, At MMJDoctoronline, we don't screw around. Ask a simple question, and you should get a simple answer, how many plants can a medical patient grow? Medical card holders can grow 24 plants or even more.

In California, under the new laws, anyone over 21 can grow 6 plants, but amazingly few people know that medical patients may grow 24 plants, provided it's the amount they need to treat their conditions and symptoms.
"A medical marijuana card is like having a cheat key in a video game. You pay less, you can buy anything, you can grow much more, all for forty of fifty bucks online, meanwhile Joe Sixpack gets Wal-Mart weed." - DJ
If you don't have a medical marijuana card, you won't be able to do much but buy Walmart weed under the new labs. Medical Marijuana Card holders are free to do whatever they like, that is consume, possess and grow the amount that is required in their medical treatment. That amount might be 24 plants, with the caveat that you may be required to justify the amount at some point.

For example, if you have 24 lush plants, and someone calls the cops saying you have a grow operation and you get the visit. You show your medical cannabis card and maybe you're good go. If the police are not up on the latest laws and you do take a trip down to the station, you direct them to the law, which states that a patient may grow the amount required to treat a given condition. In the literature, PUBmed is a good one, a study say, for pain may indicate that 20mg of THC 3 times per day is required.
"When you grow marijuana legally in amounts more than 6 plants, you want to keep quiet about it. The reason is that your nosy neighbor could be a squealer. Next thing you know, a cop is in your house, and he's not necessarily up to snuff on the new marijuana rules.." - Safety First
That means your plant production might reasonably be 60 to 100mg per day, or over a grow cycle of 100 days, that is 10,000mg. Some conditions like cancer, require much more, like 1000 mg per day for the better part of a month. This means that over a grow cycle, that is 30 g!! per month. However, that represents pretty much the maximum in the literature, and without a legitimate medical source, it's probably not going to go so well for you.

So, an average person can justify 100mg THC per day, and 10 grams over a 3 month grow cycle. If your weed is 20% THC, one ounce has 5000 mg THC in it. That means you can consume 2 ounces per month at 100mg per day, or 6 ounces over 3 months, which is something like a typical grow cycle. If your plants yield an ounce each, then six plants will do. If your medical use requires 400mg per day, that's obviously got to be for a serious condition, and the allowance will be 24 plants, that have a yield of one ounce.

At the end of the day, if you're really growing just for yourself, and maybe entertaining - giving away a bit here and there, and you have a recommendation, it's very unlikely there will be an issue, and if there is, there's ample scientific evidence to back up your medical claim, that is if you're following legitimate medical protocol.
RECREATIONAL can possess an ounce. Possess, transport, obtain or give away to other adults 21 or older no more than one ounce of marijuana or 8 grams of concentrated cannabis.

RECREATIONAL can grow six plants. Cultivate up to six plants per residence and possess the marijuana produced by these plants. All plants and harvest in excess of one ounce must be kept in a locked space not in public view at one's residence. Local governments may still forbid cultivation outdoors, but must allow it inside a private residence or accessory structure that is "fully enclosed and secure."

MEDICAL can grow 24 plants or more if their condition requires. Medical marijuana patients keep their existing rights under Proposition 215 to possess and cultivate as much as they need for personal medical use so long as they have a doctor's recommendation, regardless of the Proposition 64 limits for adult users. Beware though that local governments may still restrict cultivation via nuisance bylaws (except for the six indoor plant minimum allowed for personal use).

MEDICAL CAN USE, GROW AT COLLECTIVES AND DISPENSARIES. Proposition 215 patients with a doctor's recommendation can purchase and grow at medical collectives, cooperatives and dispensaries.

MEDICAL DON'T PAY STATE CANNABIS TAX. Tax Tip for 215 Patients: Patients who have a state medical marijuana ID card will be exempt from the state sales tax immediately, according to the State Board of Equalization. If you spend more than $100 per month on medicine, it should pay you to get a state ID card. State ID cards are available from county health departments; under Proposition 64 the card fees are capped at $100 ($50 for Medi-Cal patients).

MEDICAL CAN BUY CONCENTRATES AND STRONG STRAINS. Recreational users will be limited to strains less than 18% and access to concentrates. Medical patients with a licensed doctor's recommendation can buy any cannabis product and the highest THC strains, which may go up to 30%.

MEDICAL PATIENTS are allowed to grow indoors provided they follow the safety rules and regulations, no county may prohibit your rights to grow medical cannabis to treat your symptoms and conditions.

MEDICAL PATIENTS CAN POSSESS AND TRANSPORT, obtain or give away to other adults 21 or older one oz of cannabis or 8 grams of concentrated cannabis. Note: You cannot sell.

MEDICAL COMPLY MORE EASILY WITH REGULATIONS under the subject of "reasonable regulations" by local governments.
Local Marijuana Laws