Basically, there's just two steps:
First, create the custom document property named 'Counter' to store the numbers. You do this via File > Info > Properties > Advanced properties > Custom. You'll need to give the 'Counter' property a value, say, 0.
Second, wherever you want the count to appear, add a DOCPROPERTY field that references the 'Counter' property. You can do this via Insert|Quick Parts|Field, or you can simply press Ctrl-F9 to create a pair of field braces (ie '{}') and type 'DOCPROPERTY Counter' between them, so that you end up with '{DOCPROPERTY Counter}'. In my sample document, you'll note that the numbers always have four digits (unless you're working with larger numbers), with 0 padding. This is handled by a numeric picture switch in the DOCPROPERTY field. To do the same, you'd code your's as '{DOCPROPERTY Counter \# 0000}'. When you're done, press F9 to update the field. You can then copy & paste the field to wherever else you might want it to appear in your document.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]