15amp breaker for 9 dual outlets?
If this were a commercial installation you'd need to provision 180VA per yoke, or ten duplex receptacles per 1800W (15A) circuit.  However in residential, there is no limit. 

However that won't help.  Essentially what you're saying is you want your outlets divided into enough circuits that you never get an overload trip.  That's a tall order.  You're talking one circuit per receptacle.  I'm willing to do that in a kitchen or bathroom, but all over the house? This rapidly reaches a point of diminishing returns, and it doesn't solve the underlying problem. 

Understanding how to use electricity

It wouldn't matter if you had 2000 receptacles on a circuit.  Whether it blows or not depends on what loads are in use, what current each load individually draws, and what the total current is. 3 cell phone chargers, 9 LED lights and a table saw, that'll be fine.  A table saw and a dust collector, not gonna happen.

Overload trip

There are two possibilities here.  One is that your pressure washer, combined with the other loads on that circuit, is drawing more than 15 amps.  You will have to research that yourself, but the general concept is that you should learn what each of your loads draws.  It has a nameplate which states rated current, or you can get a gadget called a "Kill-a-Watt" which will tell you what it actually draws real-time.  

GFCI trip

The other possibility is that the appliance has a ground fault.  This is, after all, an appliance that involves water.   Somewhere you have a GFCI device which will tell you.  There is a fair chance it is not at the outdoor socket location, as putting GFCI device outdoors tends to be really hard on them, and they can protect the entire circuit from elsewhere.  Anyway, if it is a device that ONLY does GFCI - a GFCI receptacle, switch or deadfront - then you know it was a GFCI trip.  However if it was a GFCI breaker, you need to get its documentation and follow its procedure for determining whether the trip was a ground fault or overload trip.  Most of them make it rather easy. 

If it indicates a GFCI trip, move the appliance onto a different GFCI circuit, such as the bathroom or kitchen.  If you don't want to operate the appliance there, get a 3-prong extension cord plugged in there.