I usually number my cards:
1 of 1 or 1 of 3
Say I'm making a set of cards that go together.
I have 4 in a similar theme (for a swap or something)
I would number the cards 1 of 4, 2 of 4, or 1/4, 2/4 something like that.
Then for series info: Theme of Cards.
So take these:
I'd number them:
1/3, 2/3, 3/3
Series Info: Native American Series
Then, later on, if I do any more Native Americans, I can make the Series Native American Series 2.
Some people count each card they make, and some aren't in a series, they're more random. So if someone wanted to number each card, then they could put #226 or whatever number there, and leave the series blank.
Hope that clears it up a bit.
Another example is with the 100Themes Challenge.
I am numbering all mine ___/100.
Series: 100 Themes ATC Challenge