So, you're having more than twins! Congratulations. You've already figured out that expecting twins or more can put a damper on your plans for maternity clothes. While you might be thinking about everything else that goes along with a multiple pregnancy, staying comfortable in your clothes will be on your list.
The problem is that there isn't really a good way to predict how your belly will change and grow. You know it's going to be bigger than someone pregnant with one baby, but how big? In general we talk about twin moms being the same size as a full term mom of one at about 28 weeks. Triplet and Quadruplet moms experience this even sooner. What makes it different than just adding numbers, is that your babies may rest side to side, making you wider or they could stack on top of each other making your pregnant belly stick out further. This can make clothing a huge chore.
Many twin moms that I have talked to felt like they should simply wear unfashionable and huge dresses (also known as tents). The goods news is that this is no longer your only option.
When I had my twins, I went with the option of simply buying larger maternity clothes. My skin stretched incredibly well given that I had about 16 pounds of baby in there. But I still couldn't stand to have anything touching it in certain areas. So pants and shirts had to hit just right. I actually spent a pretty penny on 2x and 3x maternity clothes to incorporate my twin belly. This actually worked pretty well. Though at the very end of my pregnancy (40 weeks, 2 days), I only had one pair of pants and a couple of shirts I could still tolerate.
There are also some specialty companies forming from moms who have been there. But these don't always have items in stock, nor are they always easy to find. A local twins club group might be helpful for buying or borrowing clothes for this purpose. Internet auctions are also a good idea.
Since multiple moms tend to show more quickly you will have to figure out when to go to maternity clothes. There are products available, like the Bella Band, that help you stay in your old clothes longer, even if you cheat and don't button your pants. You can also wear longer shirts and simply pull the pants or skirts down lower on your hips.