Satoru Iwata stated the below about the New Super Mario Bros. series:
“We only create a New Super Mario Bros. title one per platform. I think we’ll probably go ahead and continue at that pace. That being said, that’s probably [Mario creator Shigeru] Miyamoto’s choice, so I can’t give you a 100% guarantee that that’s the pace we’ll continue at.”
Satoru Iwata brings out that they try to create just one entry into the New Super Mario Bros. series for each Nintendo platform. There is 1 for the DS, 1 for the Wii, 1 for the 3DS, and also 1 for the Wii U. There isn’t “a 100% guarantee” that they will continue to hold to this frequency since it is up to Shigeru Miyamoto. This may be for the good that they aren’t going to be swamping us with tons of New Super Mario Bros. by releasing one every year.