That is an interesting point, however League and CS:GO are entirely different genres, I'm not entirely sure if that is a fair comparison to be using as a facsimile foundation for how many players are in a match. 5 feels far too few; at least for battle (The only game mode worth its salt for competitive play) in regards to the feel of bannerlord. Any additional players in League would make teamfights far too hectic, confusing and difficult to balance due to the nature of character abilities. I don't know CS:GO well enough to understand why they need 5.
1. I don't really see why it would be hard to find enough good players for a team of 10, when I was competitive, clan matches in warband routinely ran teams of 6-10 and I would say all of the players on both sides were highly skilled. Of course that is entirely anecdotal and things possibly could have changed since my years in competitive but I would be surprised if they have.
2. That is an interesting point as well and I could agree with you on that to a point. I would counter that however in stating that casual audiences or amateurs typically recognize team names more than they do individual player names with exception to specific onesie twosie players that stand out to them. It's typically the hardcore fans that pay that close attention and it would surprise me if 1-5 extra players would really be a barrier to a fan. Anecdotally for me, I played League for years and can spout out dozens of different teams, but maybe only a dozen players.
3. That is a very good point and I think it would actually be the biggest challenge regarding it beyond your other points. That is why I'd like to see an in-game group/clan feature where you can manage your team or look to recruit potential talent through profiles or something.
10v10 is a stretch but I would like to see at least 6v6-8v8 for competitive battle mode as it is pretty standardized already and is enjoyable to spectate and/or engage in.