Until the next City Primary Election in May 2018, City referendum petitions require 22,771 valid signatures to be placed on the ballot. 

City Code Section 2.04.090 says that a referendum petition must be signed by a number of legal voters equal to or greater than 6 percent of the number of electors registered in the city on the date of the primary municipal election immediately preceding the date the prospective petition is filed, except that a petition signed by 2,000 registered voters shall be sufficient to call a referendum upon any franchise ordinance.

On the date of the May 17, 2016 Primary, there were 379,505 registered voters in the City of Portland.