LEDs require around 2 to 3 Volt, depending on the colour.
So with a 12 Volt supply, you could light 4 LEDs if they drop 2 Volt or 3 if 3 Volt. Use a resistor in series to limit the current. The value of it depends on how bright you want the LEDs to glow and the LED limits.
5 to 10 mA is usually sufficient.
So 820 Ohm would give you about 5 mA, and 390 Ohm about 10 mA assuming 2 Volt per LED. 560 Ohm would give you about 5 mA, and 270 Ohm about 10 mA assuming 3 Volt per LED.
To connect extra LEDs, each string will all need there own resistor. Don't be tempted to connect the LEDs in series parallel with only one resistor.
Len