Tables :: Auto Number With Command Button

Need to have a command button at form.. what i need to do is is saving a record through command button one time so produce same id for all parts like;

if i am having a field as parts and id now if i put parts as;

Parts ID

Wheel
Tyre
Rim

so at the time of button click (command button) the id number to be issued as same as 1 for all three parts, when adding more parts after few minutes or after the click then after addition of next parts the id should be 2 for all, means what i need is to have id number as respect of button click but once click then 1, then after =2 and so on..