How many amps should I use to charge a 12V battery?

For lead acid, 10% to 30% of the amp-hour rating, which is 0.1C to 0.3C. A 100 Ah lead-acid battery takes 10 to 30 amps. A 100 Ah lithium battery can usually take 0.2C to 0.5C, or 20 to 50 amps. Always stay within the limits on the battery's spec sheet.

Battery University says lead-acid makers recommend 0.3C. For lithium-ion energy cells it gives an advised rate of 0.5C to 1C, with makers recommending 0.8C or less to prolong life.

The C-rate is just current as a fraction of capacity, so 0.1C on a 100 Ah battery is 10 amps. A higher current shortens the bulk stage but not the slow topping stage near full. The battery charging current calculator finds the range for any battery size and type.