With George Russell confirmed at Mercedes yesterday, that leaves just Williams and Alfa Romeo Sauber with vacancies – Haas is poised to confirm Mick Schumacher and Nikita Mazepin will remain with the team in the coming days.
With Nicholas Latifi tipped to remain at Williams, and Valtteri Bottas' switch to Sauber confirmed earlier this week, there is precious little space left in the inn.
“I actually think we're probably missing a team or two,” Horner reasoned.
“There's a plethora of young guys in Formula 2 as well that deserve an opportunity and there's just not the seats to go around at the moment.
“So you're waiting for a Kimi to retire to open up a seat for a youngster somewhere down the chain to come in.
“So definitely, I think a team or two could be accommodated in the sport.”
Haas was the most recent addition to the sport; prior to that, in 2010, the grid welcomed Virgin Racing, Lotus Racing, and Hispania Racing Team.
Also set to join was USF1, though that proved a stillborn project which never truly got off the ground.
They were lured in under the auspices of cost control, regulations which never materialised and left them all on the back foot as a result.
A decade on, none of them exist, an ironic development given F1 has this year introduced financial regulations and a cost cap.
It's predicted that that development could lead to teams becoming profitable, therefore driving investment and interest in the teams themselves from operations such as Dorilton Capital, which purchased Williams a year ago.
However, not everyone is on the same page, with Toto Wolff suggesting the closed nature of the current grid is helping build value.
“I think Formula 1 being a closed club, like the American franchises, is what makes it the Champions League of motor racing,” he said.
Rather than increasing the number of teams, Wolff suggests teams simply field a third car, or find other opportunities exclusively for young drivers.
“What I was always in favour for is making not only the mandatory FP1 sessions, but maybe adding a race or two where those young drivers or those rookies need to race and actually be part of for the constructors' championship,” Wolff added.
“Or, even more radically, financially can be accommodated having a third car with a mandatory rookie driver, and suddenly we have a grid of 30 cars.
“The smaller teams can fund those seats with financing from the drivers or sponsors, so that could be very exciting.”
The prospect of third cars has been a hot topic in F1 in recent seasons.
While it's happened in years gone by, arguments more latterly have centred on the disadvantage it hands the teams running at the back of the field should a third Mercedes or Red Bull appear on the grid.
Another argument against expanding the grid is the division of prize money.
Historically this has been paid out to teams via two columns; one based on the finishing order of the previous season, and another to teams which placed in the top 10 constructors for two of the previous three seasons.
The addition of more teams naturally increases the competition for that pool, or a change in its distribution to accommodate new teams dilutes it.
Either way, teams rarely vote in favour of reducing their slice of F1's pie.
“Ultimately, we need probably between two and four more cars, I would say on the grid to enable young talent to have the opportunity to show [their] colours in Formula 1,” Horner concluded.
While whispers of new manufacturers entering F1 tend to perennially circle the paddock, there are no firm indications that there is currently any serious interest.
F1 next year undergoes a significant regulation change with regards to aerodynamic rules, with a similar leap expected for 2025 on the power unit side.
It's therefore unlikely that any new addition to the grid would be seen at least until those regulations have been formalised, meetings on which continue in Monza this weekend.