What amount of current should I use to charge a 12V car battery?
The normally recommended maximum charge rate is C/4 to C/5, ie.  1/4 to 1/5 of the battery capacity in Ah. 

If your battery capacity is 90Ah then 30A is C/3. The battery should handle this OK but the voltage will rise faster. Above ~13.8-14.4V (2.3-2.4V per cell) the battery will 'gas' as the water breaks down into hydrogen and oxygen. Apart from the explosion risk, this is bad because it wastes power and the battery will have to be topped up with deionised water. 

At C/3 the battery will probably reach gassing voltage at around 50-70% of full charge. To get a full charge the current must then be gradually reduced to keep the voltage below gassing level. Good chargers do this automatically, but your 12V power supply won't. 

A 12V power regulated supply will hardly charge a 12V lead-acid battery at all because it doesn't put out enough voltage. An unregulated supply will continue to charge the battery at gradually reducing current until it reaches its unloaded peak voltage, which could be 40% higher than its rating and is dependent on the mains voltage. Another problem is that without current limiting a discharged battery may initially draw too much current out of the power supply, causing it to either shut down or blow up!

For all these reasons and more, you should use a proper charger designed for lead-acid batteries. A regulated power supply can be used only if it can be adjusted to put out 13.8-14.4V and is designed to work in continuous current limiting mode.