So you're going to knit repeats of the cathedral cable block for the blanket? That'll be gorgeous.
The cast on will depend on the gauge (measure it slightly stretched over the pattern. Yes, this isn't precise, but it'll be close) and depend on the size blanket you want. For 5 blocks wide you would cast on 72x5=360sts; for 6 blocks, 432sts; for 7 blocks, 504sts and so on.
Again, depending on the row gauge, for 5 blocks in length, 98x5=490 rows; for 6 blocks, 588 rows. Better yet, knit until the end of a repeat block and measure how long the blanket is, then figure out if you want another tier of blocks.