am I correct that a character who purchases the Sorcery Background once for their Strega receives Read + 2 Minor Tesse + 1 Major Tessere?
And a character who takes the Background with its two Sorcery purchases receives Read, all 4 Minor Tesse, and any two major Tesse?
Looking at page 228 in the book, your first purchase of the Sorcery advantage (1 rank) gets you the following: Read, two Minor Weaves, one Major Weave.
It may look like you're getting a lot up front for those 2 points, but I think that's counteracted by a Strega being required to spend a Raise to use Read before she can use any other Weaves, and that all Read itself does is prep the character to use their other Weaves during the Scene.
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I read it the same as Salamanca above.
Taking Sorcery Advantage for Sorte once gives you the Read Weave, any two Minor Weaves, and one Major Weave corresponding to one of the Minor Weaves already taken.
Each subsequent Sorcery advantage adds one Minor Weave, and one Major Weave corresponding to one of the Minor Weaves the character has taken.
Technically, you don't get a Minor Weave for the fourth Sorcery Advantage taken, but I'd guess this is probably due to the fact that there isn't a fifth option for you to select from (yet).