The point with SMR is to overlap tracks like roof shingles, since the read heads can read a narrower track than the write heads can write.

In addition, the heads have multiple levels of independent actuation beyond the shared arm itself[1][2].

So as long as the tracks themselves are fairly consistent, I don't see why the number of sectors per track has to be equal.

[1]: https://pcper.com/2009/09/wd-unleashes-nested-actuator-and-a...

[2]: https://blog.westerndigital.com/hdd-magic-20tb-18tb/