How do I keep pasta from absorbing all the creamy liquid when making homemade mac and cheese?

That's the eternal mac and cheese conundrum. By its very nature, pasta is designed to absorb moisture until there is either none left to absorb or it has begun to disintegrate into mush. When I have made mac and cheese for a large group, I opted to use a slow cooker set on low to hold it during serving time. I also start by making more cheese sauce than I initially need for the dish, and hold that back in reserve. I've also found that it can help to cook the noodles in a mixture of milk and water, rather than just water. Plus, use some cream cheese in the cheese sauce. Start with mac and cheese a bit more on the saucy side than you might prefer, and get it into the slow cooker as soon as possible after you make it. Then keep the extra sauce handy to add to it as time passes. Give it a stir in the slow cooker periodically so it doesn't get overheated against the sides and bottom, and add the extra sauce as you need to. This will generally buy you a few hours of serving time- but it's not going to prevent the inevitable. At some point, you either have to concede that it will become less creamy, or add enough extra liquid that the pasta become paste. In your case, I'd probably keep the pasta and sauce separate until the last possible minute, and only join the two just before time to serve and eat. You can keep the sauce in the slow cooker by itself a lot longer than you can if you add the pasta. If you cook that ahead of time, you can give it a reheat by pouring boiling water over it, give it a good drain, and then add it to the sauce in the slow cooker.

I find you need a lot more sauce than you think . Are you making a bechamel base?