Yeah
You'd think it would be the field code autonum...
You want to stay away from Autonum, especially if you are migrating between different versions of Word.
Use SEQ.
So let's say you want to start the first number at 500.
Use the menu and choose: Insert/Field Code.
Indicate SEQ. And use the following X \r500
So the code will look like this in the document if you expose codes:
{SEQ X \r 500 \* MERGEFORMAT}
X is just a variable name. Don't have to declare it or anything, it's just so if you have different sequences of numbers, it knows which one to increment, so you can have multiple series of sequential numbers. You can change X to be anything you want. Even a word.
\r means to reset the number count beginning at... In this case 500 as that's what I typed
Now, if you want to increment, then you use the following field code:
{SEQ X \* MERGEFORMAT}
Sorry if this isn't written very linearly. It's 2:45am here!
Hope this helps