If you plan out your garden well, you can figure out how many plants to purchase before hand at the Seattle Tilth Early Edible Plant Sale - March 19th! check it out at http://seattletilth.org/. The good thing about buying plants there is that you can get plants that should be grown this season in our area such as peas, kale, chard, spinach, broccoli, chives, etc. Pretty much anything you buy at this sale, you will be able to put in the ground now or very soon, then you can go back in May to the next edible plant sale and purchase your next round of plants for the hot weather as well as more lettuces, etc.
A few things to consider:
1) How much can you or your family eat of a particular crop in a week? Ok, after you decide, plant a few for the slugs, etc. For head forming lettuces that are all going to be mature at the same time, I usually try to buy at least one kind of lettuce seeds that I like (or a lettuce mix) each season. I plant most unique lettuces I can find at the sale, then buy a reliable seed lettuce to sow in between the plantings so those plants that come up will replace the older ones. I make my lettuce seeds last a long time by putting only a small pinch exactly in the spot between the bigger plants and I have less thinning to do in the end.
2) How much space do you have? If you have good soil to start with you can put more plants than you think in the bed, especially when you can harvest lettuce at any age. If you do not, amend it with compost and/or amend with a nitrogen rich natural fertilizer. Make sure the leaves are not touching each other or especially the ground. If you stagger the plants by height so tall growing plants are in the back (North side) of the row, and short plants are positioned in front the plants will stay healthier, and there will be less chance of rotting leaves. I have grown kale really close together and it stays small and spindly, but makes a good tender stir fry mix, and also I have grown a few dino kales for three years (and they really did look like dinosaur chicken legs!
3) Leave space for your summer crops! You can grow things like peas where you will eventually grow tomatoes because they will probably be done by the time the tomatoes get big enough to matter (depending on the year, of course). You can also get creative with it and put food crops in containers with your spring flowers or herb containers, and you can also plant seeds indoors on a window to get them started, but use a cool part of the house so they are not too dependent on the heat- seeds like warm moist medium to sprout, but they usually do fine outdoors soon afterwards.
4) Watch the watering- We usually have a short time in the spring when we have a mini drought, so make sure the bed in moist and the seeds don't dry out before they sprout. But, do not water other areas until you check the soil. Dig a shovel in the ground nearby and see if there is moisture where you want it. If the seeds have sprouted, chances are the roots go way farther than you would think. Most roots have small hairs that extend way down. the more they have to reach for water, the deeper (and usually stronger) the roots will be. To check the soil easily, grab a handful and squeeze it in your hand. If it sticks together it doesn't need water (and may need compost added) and if it falls apart, you need to water.
I plan on making a couple more raised beds fitted with old windows to help the sun warm the plants and grow faster. During the spring I will use it as a seed starting bed, then transfer the plants out to grow in the regular beds or in a warmer hoop house in the case of my tomatoes and eggplants. Then, in August or so I will start more lettuces, spinach, and kale to hopefully last the winter.If it is hot, I may need to use shade cloth to start the seeds. I live in a low area with plenty of moisture, so I haven't had much luck with carrots, but I plan on starting a just slightly raised bed to grow yellow carrots in the spring and see if I can over winter some carrots, too.