Series circuit.
Say for 10 Red Leds (at10mA) makes 10 x 1.7 is 17 volts.
Use 24 volts DC supply, then a 700 ohms resistor is required.
On AC 24 volts an other string of leds can be wired antiparrallel so 20 leds can be connected with one 700 ohms resistor (680 R)
a 1/4 or 1/2 watt resistor will suffice.
Other colour leds have different voltages for running and vary between 1.7 and 3.7 volts, check led data sheet.
As long as led current for normal leds stays below 20 mA's there will be no problems.