In western WA they never want for running water, though this year's
record-low late winter snowpack in the Cascades might make it a little too interesting if it doesn't pick up before summer, which may affect late summer power capacity too, given the prevalence of hydro driving the PNW grid.
If natural gas is available, a 50 gallon gas-burner + drainwater heat recovery could pretty much keep up with 2 simultaneous shower with modest recovery time between the next batch.
If the space is available, an 80 gallon heat pump water heater + drainwater heat recovery unit would do OK and would use even less power, but the recovery periods would be as long or longer than what you're currently experiencing. It's a pricey piece of hardware (about $2400 for the well regarded Stiebel Eltron Accelera 300, $1800-2K for GeoSpring et al), but with your hot water use it may eventually pay off on power use, but not nearly as fast as drainwater heater recovery would. They're all pretty tall 6' or taller, and they're not exactly quiet, but they help dehumidify the basement (assuming it's installed in the basement.)
There may be rebates available for either heat pump water heaters or drainwater heat exchangers your area, depending on which utility is serving you. (PSE kicks back $250 for any drainwater heat recovery heat exchangers with better than 42% return efficiency, $300 for heat pump water heaters.) Got a ZIP code, or the name of your electric utility?
The 80 gallon heat pump water heaters are rated between 75-85 first-hour gallons, but in combination with drainwater heat recovery it's substantially more than that. A standard 50 gallon tank is good for about 55-65 first hour gallons (as are some 50 gallon hybrid heat pump water heaters), a standard 40 gallon unit is good for maybe 50-55 first-hour gallons. From a showering-use-only perspective a drainwater heat recovery unit of sufficient size will get 65+ first-hour gallons out of a 40 gallon electric, but not more than 75. At typical shower head flows you're probably looking at about 15-25 minute of showering time in that first hour with your 40 gallon tank, which is about 2 showers or 3 fairly short ones. With drainwater heat recovery you should be able to get 3-4 decent showers before having to wait, with a shorter waiting period. With an 80 gallon tank (standard, or heat pump type) you'd get in 4-5 showers, but in combination with drainwater heat recovery you'd be able to shower all 6 people in rapid succession without much concern about running short.
Unless you are heating the place with electricity, water heating is easily the largest chunk of your power bill- it could be more than half. (I'd need a lot more info to be able to assess that.) With drainwater heat recovery + heat pump water heater you'd be using less than 1/3 (and possibly less than 1/4) of the electricity for water heating than currently.
If you're not all showering at the same time of day (3-4 in the AM, &/or 3-4 at night) you may do just fine retrofitting the heat recovery unit and deferring the decision on the water heater itself. If that works, it's probably worth bumping to 50 gallon GeoSpring (about $1000 at box stores) if there are subsidies available, since it has the same first-hour ratings of standard hot water heaters, but uses half the power. The ~$300 difference in unit cost between a subsidized HPWH and a standard 50 gallon unit would be recovered in lower power bills in less than 5 years, maybe under 2 years at typical WA electricity prices, even if you have the drainwater heat recovery in place.