[excessive]

I don't mean this the wrong way, but since you know one instruction is sufficient, the next smallest number is two. Maybe by RISC, you meant a small orthogonal set? Load/Store/ALU ops separated? So ADD and NAND work on registers with transfers using LOAD and STORE? Seems like you'd need a branch, unless the instruction pointer was one of your registers.

Brainfuck has 8 ops, 2 of which are for input and output. It's Turing complete, and the operations are super simple, but I'm not sure it's RISC-ish.

https://en.wikipedia.org/wiki/Brainfuck#Commands