In 7th Edition I ran squads of 10 Berserkers in Rhinos, in 8th Edition I ran squads of 8 Berserkers in Rhinos (often with a buffing character), in 9th Edition I've run both but still lean towards 8 (though never with buffing characters thanks to how precious HQ slots are now). One thing I've never tried but see put forth quite often by B&C fraters whom I perceive to play in more competitive metas then me put two squads of 5 in each Rhino for more power weapons and presumably greater board control / movement versatility.
I would like to gauge how popular each squad size is in and specific reasoning / case studies as to why people go with the unit size they do.
I generally run with 8 because it is fluffy, and bumping my 2-3 squads to 10 is basically the difference between having another Daemon Engine or not or having extra ablative wounds on a Melta bike squad, etc. If I have extra points leftover I may use them to buff a squad from 8 to 10, but I have never really noticed a difference performance wise.
In 8th I also used to run them with a Chaos Lord (solidifying 8 instead of 10). I always found Chaos Lords more useful then the more popular Exalted Champions because I could more safely have them run off and do their own thing thanks to the Invulnerable Save (the To Hit aura seemed to be equally as effective as the To Wound aura as well). In fact, I rarely ever kept the Chaos Lord in cohesion with the Berserkers for long after they jumped out of the Rhino for that reason.
Even still, with that said, I haven't run into a situation yet in 9th where my Berserkers jumped out of the Rhino and I found myself wishing I could split them in half to go into two different directions largely due to Maelstrom no longer being a thing (or at least not a thing in my area, I don't know if people elsewhere still play Maelstrom). For those who don't know, Maelstrom was an old format where your objectives were randomly generated at the beginning of your turn so you had to adapt your strategy on the fly unlike the largely static objectives of 9th.
I look forward to your thoughts and stories.