Ideally I would focus on a single team organized around a single product.
The reality is most engagements I end up helping an organization improve while helping multiple teams. The reasons behind this are many though usually budget is a key driving factor.
The challenge becomes how to get the team to engage with practicing new skills on a regular basis when you yourself aren’t available on a regular basis.
So thus far I have had the best luck with finding and encouraging a champion of sorts that is willing to try new things and is able to get others to play along with them long enough to pick up the practice themselves.
I’m interested to hear others expereiences!