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1) There is no limit to the number of blending data sources you can use. It will impact the performance though...
2) No, a blend will always be a left join. You can not change this behavior. You can try using Tableau Desktop 10 where you can cross database join.
There is a very good webinar on data blending that has been recorded - see the link below. The webinar is also offered live - see the schedule on the Tableau site