You say only once without specifying when it should reactivate, so if you want EXACTLY what you said you should deactivate the flow and never ever reactivate again. No second flow needed (too bad “delete flow” isn’t available as a card) 
So, assuming you want something useful instead, when should it reactivate? That is what was asked: what do you want to achieve? E.g. if you turn on a light based on the temp: add a “when the light is off” and you’re done.
I’m guessing that without a second flow it will prove really hard. Kind of reminds me like the good old days, when people tried to write entire programs in one single line of Basic. Doable, but not always recommendable.
If you want a oneliner, and do not tell what you want to achieve, I’d suggest looking to HomeyScript. But even then there MUST be a situation, implying a trigger, implying a flow to activate it again.
Two flows is not nessesarily bad, it is most likely the cleanest best performing way to get wat you need in most situations. Other options often imply Homey needs some background work (timers and such), which is less efficient and may obfuscate what is going on if the situation changes inbetween.
