EE Stock Purchase Plan - max # of shares?

[Guest Beth N]

I've been told that in an ESPP the max # of shares an ee can purchase is set in stone as of the first day of the purchase period (equal to the $ amount deferred divided by the lesser of the purchase prices offered throughout the purchase period), such that if the purchase price drops, the ee gets the benefit of the lower purchase price, but can't purchase more shares than they would have purchased at the beginning of the purchase period under the higher price. The difference is refunded to the ee.

Does anyone have a source that will confirm this principle? Many thanks.