Three races on the calendar released last week have asterisks against them - South Korea, New Jersey and Mexico - and the prospects of the first two at least have been called into question.
Most teams are reluctant to go beyond 20 races, mindful of the added expense and the burden being shouldered by key staff such as engineers and race mechanics who cannot easily be rotated.
This year's championship has 19 races, with the record standing at 20.
"I think we all recognise that 22 races is beyond the limit, the strain that it puts on the team and the entourage that follow Formula One," Horner told reporters ahead of Sunday's Korean Grand Prix.
"If you are doing 22 races in economy class, flying around the world and you are going a week before the race and coming back a day or so after, it's a long, tough season.
"For me the ideal number is 20, that I think is saturation point, but I can understand why he (Formula One supremo Bernie Ecclestone) is pushing to get more venues."
The calendar approved by the International Automobile Federation includes a rare 'triple header' of Monaco followed by New Jersey and then Canada on successive weekends, which poses an added logistical problem.
"Of course it's feasible. But it's expensive," said Horner of that challenge. "Let's see. The calendar's changed a lot over the last few weeks and I'm sure before the end of the season there will probably be a few more tweaks to it."
Horner said New Jersey, which was meant to make its debut last year but postponed for financial reasons, was on the calendar so "we have to assume it's happening".
McLaren's Martin Whitmarsh was less convinced, however, even if all the teams want a race on the U.S. east coast.
"As I understand it, there's not much going on there," he told reporters.
Mexico has been pencilled in for the penultimate race of the season on Nov. 16 and the country's top racer, McLaren's Sergio Perez, was confident the Mexico City organisers would not let anyone down on their return after a 22-year absence.
"I spoke with (FIA race director) Charlie (Whiting) the other day and he was very optimistic. He thinks they have to do a few changes to the pit lane, and the layout a bit in a few corners," he told reporters.
"They have changed the schedule of the circuit so they are not racing on it any more this year because they start to rebuild it."
South Korea is due to switch to an April 27 date but the race has made heavy losses since its debut in 2010, with sparse crowds, and local promoters are seeking to renegotiate their terms with Ecclestone.
Few in Formula One would mourn its passing, despite the country's economic importance.
"We want to improve the current contract with Formula One Management. We are still negotiating with Mr Ecclestone," race promoter Won-Hwa Park told Sky Sports news this week.
He put the chances of the race remaining on the calendar next season as only "50/50" but added that South Korea wanted to keep it if possible.
An official banner at the entry to the circuit on Friday declared: "Thank You Mr Ecclestone for 2013 F1 Korean Grand Prix".