Why do we need two gluons for the decay $ϕ \to 2K$?

The Feynman diagram for the decay $$\phi \to K^+K^-$$ usually depicts two gluons. (This can be seen e.g. on Wikipedia).

Why do we need two gluons, instead of just one?

• I've removed some comments that answered the question, and replies to them. Please post answers as answers, and use comments to refine or improve the posts they are attached to. – rob May 17 '20 at 4:45

If you are thinking of the figure in your question, (which is missing the antiparticle signs) to eliminate one of the gluons and take (for example) the anti-up to match with the s quark , it is again energy/momentum but in terms of probabilities, the unti-up would have to have the correct energy momentum vector to match the mass of the $$K^-$$ with the s quark, very improbable.