This page lists the population limits that have to be reached in order to build a certain amount of industrial buildings.
On the July 4, 2013, the rules were changed so that buildings in storage suddenly counted towards population limits. This made it difficult for players with a lot of existing buildings in storage to buy new ones, as they first had to make massive population increases.
In a later update the same month, the population requirements went down somewhat, presumably in response to players complaints. The "confirmed" column here indicates whether the number has been confirmed since that happened.