contribute nondeductible money to traditional IRA and convert to roth each year, what should IRA basis be?

Hello

I am making nondeductible contribution to traditional IRA and convert to roth each year. What should the total basis for traditional IRA in line 2 from form 8606 be? Should it be 0 all the time? Or should it be the total amount of  nondeductible contribution I made from previous years? (Increasing each year, for example : 6000$ -> 12000$ -> 18000$ ..) Thanks!