Just one yarn over. Otherwise it would say yo, k1, yo, k1 etc.

thank you for clarifying that for me.

When a pattern has asterisks, like you showed above, it means to repeat the stitches in between the asterisks. So, with your example of *yo k 6* you would "yarn over, knit six, yarn over, knit six, yarn over knit six" until you got to the end of the row (or round, if you're knitting something on circular needles).

Sometimes a pattern will say something like:

*yo, k6* 3 times, p1

So, you would "yarn over, knit six, yarn over, knit six, yarn over, knit six, purl 1" and continue with the rest of the instructions.