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spud= singular
spuds= plural
"I bought 2 pounds of spuds yesterday."
Tater is also slang (southern US) for potato but is not used as a substitute for "couch potato". Spud and tater are both used...

Also "spud" is used in place of "couch potato" (spud is slang for potato).
"He's a real spud." (ie lazy, a couch potato)
"Don't be such a spud!" (don't be lazy like a couch potato)
"Couch potato"...