The Eldritch Cannon lasts one hour, and every bonus action, the Artificer can give multiple creatures within a radius 1d8+int temp HP. If the characters use this an hour before expected combat, twice every 6 seconds for one hour, they gain at minimum 1, at maximum 13 temporary health. Have I misread something? Or does my party get at least 600 temporary health because my 20 int artificer thought an hour ahead? I assumed that it would cost something, but I cant find anything. This can't be right.

I am the DM, haven't played Artificer, so I don't know how Artificer works beyond what my players tell me, and the limited research I've done. 

Temp HP don’t stack. If you have one million sources of between 1-13 temp HP, the maximum Temp HP you could ever get is 13 because that is the highest potential number of temp HP from any of those sources. If you get to one million and one sources, and that last source is 13 or lower, no change whatsoever. If that last source grants you 15 Temp HP, your number of Temp HP suddenly jumps to 15.

Temp HP doesn't stack like that. Best he can do is let them reroll until everyone gets an 8 on the die. From the PHB:

 If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.