Will there be enough provisional ballots?

[Eric J in MN]

http://story.news.yahoo.com/news?tmpl=story&cid=519&ncid=519&e=4&u=/ap/20040122/ap_on_re_us/election_changes_1

"And one change will assure voters in all but five states a chance to vote, even if the validity of their registration is challenged at the polling place. Provisional ballots will be available, so a voter can still cast a ballot while their claim is investigated. If the registration is later discovered to be legitimate, their vote will count.

That change alone is estimated to capture the choices of between 1.5 million and 3 million voters whose ballots were lost in the last election."
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The writer seems confident that any voter purged from the rolls will be able to vote with a provisional ballot from now on.

But what is stopping a precinct from saying, "you were purged and we're out of provisional ballots so you can't vote?"