Special Stitch in Crochet: Double Crochet 3 Together Instructions

Special Stitch
dc3tog = double crochet 3 together - To make a dc3tog, work 1 dc in each of the next 3 sts but leave the last loop of each dc on the hook, resulting in 4 loops on the hook. Yarn over and pull through all 4 loops.
The quantity of sts is a multiple of 10 + 1 + 2 chain sts increase.
Row Instructions
1st Row
Sk 2 ch (count as 1 dc), dc into the next ch. *dc into each of the next 3 ch, over the next 3 ch work dc3tog, dc into each of the next 3 ch, 3 dc into the next ch; repeat from * ending the last repeat with 2 dc into the last ch, turn;
2nd Row
Ch 3 (count as 1 dc), dc into the first st, *dc into each of the next 3 dc, over the next 3 sts work dc3tog, dc into each of the next 3 sts, 3 dc into the next dc; repeat from * ending the last repeat with 2 dc into the top of the turning chain, turn.
Repeat the 2nd row.
How do I calculate how much yarn I wil need?
How many chains do you start with?