By law, companies can only offer 10 standardized Medigap policies known as A-N. The plans are A, B, C, D, F, G, K, L, M, and N. The plans previously labeled E, H, I, and J are no longer sold. An insurance company does not have to carry all ten plans. Some may only carry a few, while others may offer all ten.
You only need one policy. Each policy builds on the previous one. Plan D includes all the coverage options offered by A, B, and C, plus additional options under Plan D. Plan E, provides everything Plan D does, plus additional coverage, and so on. It is illegal for an insurance company to sell you more than one plan.