Russia, who beat England and joint bids from Spain/Portugal and Belgium/Netherlands last December to win the right to stage the football extravaganza for the first time, had originally proposed 16 stadiums in 13 different cities.
World governing body FIFA usually considers 12 venues as an optimal number for the 32-team tournament.
"Twelve is a more practical number because the more cities you have, the bigger the cost is of staging the tournament," Mutko, who is also a member of FIFA's executive board, told reporters.
"We haven't made the final decision and in theory all 16 [original] venues still have a chance to be selected. But in the end we would have no more than 13 stadiums."
Mutko also said that Moscow's 84,000-seat Luzhniki Olympic stadium would likely host the World Cup final and one of the semi-finals, with St Petersburg staging the other semi.
"Moscow alone has three different stadiums, plus another one just outside the city," he told Reuters at an international sports forum, which concluded over the weekend in Saransk.
"I think it's just too much. We shouldn't have more than two World Cup stadiums in Moscow."
Moscow's second arena would likely be chosen from the two stadiums which are being built by Premier League clubs, Spartak and Dynamo, Mutko said.
FIFA's Secretary General Jerome Valcke was impressed with Saransk, an industrial city with the population of about 300,000, after visiting it on Friday.
"It's a nice city, very compact. I think it could host some of the group matches," Valcke told Reuters.
Saransk, located some 600km east of Moscow, is considered an outsider to make the final cut.
The final decision about the number of venues for the 2018 World Cup should be made by FIFA in March 2013.
