Uln2803 - as relay driver

[sayzer]

No need for the resisitor between the Pic pin and ULN pin. Direct connection.

And for your question, for 12 relays, use two ULN2003 instead of ULN2803.
You will have 14 outputs.
And, do not trust the diodes in ULN chips, use external diodes accross the relay coils.

[Heckler]

You can also parallel outputs on the ULN chip to achieve more current handling. Each output will sink 500mA, so two in parallel will sink 1 amp. One post I read suggested stacking and soldering two DIP ULN chips, one on top of the other to achieve this increased current while still maintaining the same number of outputs.

[sayzer]

There is also TPIC595 "POWER LOGIC 8 BIT SHIFT REGISTER".

With just 3-pins on PIC, you can drive as many outputs as you want and the load per output is 500mA.