good day po Sir!

i bought my 2nd hand car (toyota xl 94, 12 valves, 1.3) 3 years ago, since then i haven't replace yet the spark plugs..just wanna ask,what is the frequency of changing spark plugs?are there any signs in performance that will guide me that it's due now to replace it?i'm sorry i just have no idea regarding this.trying to maintain my car in good condition
also i wanna ask if how many spark plugs my car has?do you have an idea sir how much is the price per pc?

thank you sir!