So you want to do a group project of a copyrighted design? Because you wish to sell/raffle the results of a copyrighted pattern, you are wondering if you need to ask permission and how many copies of the pattern do you need to buy?
First place to go is to their website if they have one and look for their use policy. Some designers have many specific requests which are largely unenforceable. Many places say "only for personal use". But what exactly does that mean? Some would say that it is the physical pattern that you are buying, the finished quilt is a different object and so long as you aren't reproducing and selling the pattern you are fine. As for the number of copies, I would say that you are making one quilt so only need one copy.
Is the designer a selling point of the project? That is, is it a distinctive style or look or do you achieve value by using that pattern over any other, other than a desire to use it?
There are a lot of different issues relating to copyright, they differ slightly from country to country but there are international agreements. And there are reasons why we have lawyers, there is usually more than one side to any issue. I have a minor amount of background in copyright, having worked in the creative department of an advertising agency as well as on public projects. Frankly, I find it an interesting subject.
My belief and understanding of the subject is this: If you can draw it, under "fair use" you can make it even if it is what I call a "direct lift" of someone else's design down to the fabrics chosen and quilting patterns used. However, if I'm going to copy someone's legitimate original work, I personally am going to buy the pattern to have that right. For example, although I still haven't made it, I drafted out Elizabeth Hartman's Hazel Hedgehog pattern when it first came out. I could make it myself, but I know it isn't mine and so I paid Elizabeth for her cleverness and ingenuity.
You can even sell that one piece as your own work/product of your own hands even though you can't claim artistic integrity in that case. You should ethically credit the originator but don't have to under most circumstances. Many shows allow you to show copyrighted patterns and/or kits even because it is your workmanship, but some do not. Always read the requirements and err on the side of caution.
Your case is slightly different because you are making it essentially for sale and there are ramifications I don't have to face when making things for myself.
Speaking of requirements, my state (Washington) is one where there are very many hurdles to raffles including 501(c) status and filing paperwork pre- and post- raffle.
But, you can't copyright a log cabin quilt. You can, however, copyright the directions and techniques you write yourself to make your project.
A minor change can be enough to make something original. We've been discussing some of the issues in this thread:
Needing Help Finding A Quilt Pattern