As mentioned in the prerequisites, you need a core team of code craftsman to kick off a tribe. Typically this would be an experienced XP team that already work well together. If you have a scrum team and want to make them an XP team, you may need to hire an XP Coach or seek the help of a consulting firm. Contact Us for a recommendation. (Or you could experiment by just jumping in with a large tribe too. Not sure. Let me know how your experiments go.)
XP teams can be larger than Scrum. I think this is likely due to the fact that they are pair programming. A scrum team is 5-9 developers whereas an XP team can be up to 20 (though 12 is more usual).
If the rate of output of this team needs increasing then add to this tribe. But only add new developers at a growth rate of 20% and wait until that 20% has settled in before adding more again. The process will gain momentum as the 20% growth rate will compound as the tribe grows.
The theoretical maximum size of a tribe is 150 developers. Why 150? This is Dunbar's number. It is important from tribe members to maintain a relationship with the rest of the tribe. Hence, Dunbar's number.