how come their are no 7 cylinder engines
It’s all about size, balance, and output, and perhaps a few other factors.
An inline 7 is a long engine. It would be hard to fit under a cars hood. Very few manufacturers make an inline 6 because of this. If they have a hard time fitting 6 cylinders under the hood, 7 cylinders is even harder. They switched to a V6 configuration on many of their vehicles because it is a more compact design in length.
Also, an inline 7 is inherently unbalanced. An inline 6 is naturally balanced. Being unbalanced means needing counterweights and more complicated design. On top of that, you can’t really easily have a V7 configuration. That is even more complicated a design even though it is compact. Usually V configuration is an even number higher than 6. V6, V8, V10, V12/W12.
The output of a 7 cylinder isn’t too much greater than a similar I6 or V6 or too much lesser than a V8. And those designs are preferable because of the reasons above.
this isn’t an exhaustive list, but some of the reasons that make a 7 cylinder not as practical as some of the other engines for cars.
And, in addition, there are a LOT of radial engines with cylinders ranging from 3 to 18 (43 liter displacement)
https://en.wikipedia.org/wiki/Radial_engine
Here's one used in both aircraft and armored vehicles
https://en.wikipedia.org/wiki/Continental_R-670
There are, but very few were made. Mostly found in some marines and radial engines but never been used in automobiles because it's too complicated and unbalanced when something more balanced like inline 6 or V8 since they're even number of cylinders.
