While sometimes a larger intake can signify a higher likelihood of a higher availability of formal training programmes and organized support structures, this is not always the case (with a firm like
White & Case it definitely is, but at a firm like Kirkland a lot less); and furthermore it is definitely not indicative of overall training quality in my view. The risk that comes with a larger cohort size consists of each trainee getting less visibility and becoming more of a "cog in the system", which is exactly what many people who apply at US firms seek to avoid. As such, I would not say a trainee cohort size of under 10 is a bad sign. I would rather just infer that you will likely get a lot of visibility and responsibility (perhaps more than at some US firms with larger cohorts) and that you should also likely not expect much in organized support (perhaps somewhat less than at the US firms with larger cohorts). Whether that is good or bad will just come down to your preferences.
As for the questions about the best US firms for training, I can only comment based on the firms I have had the opportunity to interact with and research, so I will undoubtably miss a number of great ones. The ones I can say I have seen some quite positive reviews for are
White & Case (although I have been told it is more similar to MC training than US training in many regards nowadays),
Willkie and
Davis Polk (who are examples of places with small cohorts but where the cultural elements I mention seem to be working well), and
Latham (which I understand comes with a more hands on experience than a
White & Case but less so than firms like
Willkie and
Davis Polk).