IMHO. One in the engine compartment, (automatic, pyrogen or halon replacement) One in each cabin, (dry powder), one (dry powder) plus fire blanket in galley, away from the cooker. One in the companionway, to fight your way out. One outside engine compartment, (dry powder) in case auto one doesn't, auto I mean! Bucket with lanyard. All 2kg extingiushers.
I have one in the after cabin, one in the maion cabin, handy for galley and engine, and one in the cockpit locker. I also agree that an auto in the engine compartment is a good idea. Compare prices in Halfords or auto shops before buying from chandlers, they tend to be the same extinguishers for less money.
Must agree that an automatic system should be fitted in the engine compartment, the fire may be well alight before you know you've got one. You can't buy halon any more and if anybody has it fitted it must be removed by the end of this year. Halocarbons are what you want to be looking at for the engine room. Dry powder is common and quite cheap, but on the down side it makes one hell of a mess it also has no cooling effect so the item on fire could re-ignite if it's still above its ignition temperature. Depending on what is on fire, a pan/ bucket of water might be a better choice and it won't make the mess a dry powder one will. CO2 is also usefull to have on board for fire in electrical equipment, could also be used through an opening into the engine compartment if the auto one fails to knock the fire. Now suppose you get a hole/split fuel pipe, fuel spills onto a hot exhaust and in a few seconds you have burning diesel all over the bilges, you need a foam extinguisher (AFFF)
Get a few, space them out around the boat, sit down and make a fire plan/risk assesment and brief the crew.
There was a good article in january PBO if you can get hold of a copy.
Can never have too many extinguishers on board, but a word of caution!...... having had the occasion to use a dry powder on an electrical fire(automotive not boat,might i add!) it was found several weeks later that advanced corrosion had set in on all electrical wiing and terminals (as well as relays and switches) requiring considerable rewiring wok to be carried out!!! A discussion with a local fire extinguisher firm revealed that the powder used is fine --- until moisture comes into contact with it at which point it becomes corrosive!!!!!!!!!!!!! So be advised that if you should have to use one (as indeed you must should the occasion arise) be prepared to spend plenty of time removing ALL traces of powder. Hope this is of help to you. Safe cruising, Scorch.