We're going to have more than 5 companions. At the very minimum 8. (Adam is writing A character and one writer is said to write two linked ones, so at least 3 more, as I understand it.) It's been said though it won't be as much as in BG1/2.
Sources:
Panel From Hell, 1:25:45 - what's coming during EA.
Yesterday's interview (link on Swen's twitter) - somewhere in there is included a mention that the current companions lean towards evil (can't remember where it was first stated, Dropped Frames interview perhaps?); it's a 30 minute watch, I recommend it as Swen talks about EA plans and also confirms dialogue will be heavily tweaked (something people here suggest a lot, for good reasons).
As for my personal opinions/estimations, I think 10-12 is likely. 12 would be great as you can have one per class. (More would be even better, of course...)
