Just one
Source. There are lots of discussions at places like Battery University, 12 volt side of life.
If you check the SG and say one or two cells are very low they could be bad. If all of them are low but close to each other an equalize may help.
Some solar controllers have the ability to go to those voltages. Some aftermarket converters can and some use automotive chargers that may reach it. I use a Meanwell brand Wich is manually adjusted to go from about 13 to 17 volts.
Pay attention during the process and don't start it and leave it alone.
A pendulum type hydrometer is available at auto parts store for about $12. If you want laboratory grade get a Prease(?)