Wrong answer
Ok, I'm about 99.99% sure you're planning on shooting Servcie Rifle. In that case, the mag must have the same dimensions as a 20 or 30 round USGI mag. Might as well use real USGI 20's because they're so good, as opposed to some company's abortion magazine. Not only that, but I won't support anyone cutting and welding our limited supply of 20 and 30 rounders to make 10 round Klinton mags.
Now to your question. I use 5 mags for match use, and I carry a spare for "just in case." I have two mags with a big "2" on them in white paint marker for the 2 round portion of the rapids, and another 2 with a big "8" on them. I load them the night before. Of course on an 80 shot match you'll have to reload your rapid mags once. Plenty of time for that. The last mag I use for standing and slow prone. The follower never even gets depressed, but you should have one that will always lock the bolt back for this job.
You may use 30's, but generally they're a PITA because they often get in the way for standing, and for sure they'll be in the way for sitting and prone. 20's are perfect for the job.
Jon (JC121) might be able to get you some good prices on 20's in you're interested.