Most mission calls being issued now ask for one or two suits (two-pant suits are a must) and then a couple of items down, you'll see that they ask for washable dress slacks. A common misconception, especially for first time missionary parents, is that the pants that come with the suits are enough. That since the suits come with two pants, and they ask for 3-5 dress slacks, then pants are done!
When that happens, we often hear from missionaries in the mission field (or their moms) and usually what they tell us is that after the first summer out in the field, for some reason the suit pants don't match the suit coat anymore. The sun, even in moderate climates, takes it's toll on those suit pants they have been wearing without the coat for the past 3 to 6 months. This issue can be avoided entirely by leaving the suit pants with the coat when you aren't required to wear a suit and get separate dress slacks for those months.
The concept is simple: you wear four pants (from your two 2-pant suits) in the winter, and then you have four dress slacks in the summer. Yes that's 8 pair of pants, and yes you will need all of them.
Wearing 8 pair of slacks for two years (730 days) means every pair of pants will be worn about 91 days before the end of your mission. That may not sound like a lot, but considering the heavy wear they will encounter every day they are worn, it adds up. Taking just the four pants (assuming you get two 2-pant missionary suits) means each pair gets 181 days of constant wear. Ouch. Let's not even talk about the bargain suits that only come with one pant... (how many pieces of clothing do you wear 365 times and are still in good shape?)
Save yourself time, and over the long term, money, by buying as many dress slacks as the call asks for right at the start. You'll be ready to go, you won't have to worry about getting new pants sent from home, and your suit pants won't be fading day by day during those white shirt and slacks only months.