I could be more precise if I had a link to the pattern, because then I would know what we talk about.
Mostly what you do in intarsia: you have a few rows of knitting done (ribbing, cuff, straight, whatever your pattern says, mostly anyways).
Then the intarsia part begins: every block of color gets a bobbin of its own. Your question is: what is a block of color: well, you see that easily by going through the chart line by line (row by row): every color change marks a new block. So every time you go from one color to the second: new bobbin.
Now, if I knew your pattern, I could tell you more... If your letters are - let's say - just one or 2 stitches wide for each line in the letter, I might take a new bobbin for every line of that letter, but I would strand the main color behind (call that "fake intarsia" if you like) For true intarsia you would start a new bobbin of main color there as well.
Oh, you do not always start a piece of knitting with a purl row.
The pattern says what side (Right side or Wrong side) you start on. Either is fine.