Short answer is 0C
Longer answer…
This will depend on the type of beer you’re cooling (eg lager needs to be cooler than bitter), the ambient temperature where the kegs are stored (warmer beer needs to be cooled more), and the length of the cooling coils in the glycol bath (eg 3m, 6m, or a combo, longer=cooler).
Assuming you have 3m coils (cool the beer by up to 7C) and 6m coils (cool the beer by up to 15C) you should route your ales through the shorter coils and lager through the longer coils. This would result in say, 5C for your lager and 12C for your ale.
If the ambient temperature where your kegs are stored is 18C then a glycol bath of 3C should theoretically allow for the full 15C cooling. I’d go at least a couple of degrees lower because there are always losses. You can adjust the setting up or down based on your own temperature measurements from there.
If you want to take the glycol bath below 0C you should add monopropylene glycol (food-grade antifreeze) to the water in your glycol bath. There are tables on-line that tell you what ratios are needed, I use 40/60 glycol/water which allows me to go to -20C.