Why does the current in an electric circuit decrease when more bulbs are added?

If you add one extra bulb and the voltage remains constant, then you have doubled the current drained from the regulator.

12 Volt and One 12 Watt light bulb drains 1 Ampere Current.

12 Volt and Two 12 Watt light bulbs drains 2 Ampere Current.

However:

If having a 24 volt power source and you add two 12 Volt 12 Watt in serial, then you still only drain 1 Ampere Current.

NOTE:

Wattage and Voltage of bulbs may be different even if the sockets are the same.

Lower voltage on the bulb will increase the current drain, if voltage is a lot lower it might cause the circuit delivering voltage to burn out or blow a fuse. It can also quickly burn the bulb, sometimes in a fraction of a second.

It will however do little damage to add a bulb with higher voltage than the circuit is designed for. You will then only observe that you do not get the light you might hope for.

Total Current/Ampere= Combined Wattage divided by Voltage

Total Wattage = Combined Current or Ampere multiplied by Voltage.

In simpler Words:

If you double the bulbs, twice the current is drained from the battery