There are 18 seats in Section 233B, Row 21. As you look towards the field from these seats, seat 1 is on the right aisle and seat 18 is on the left aisle.
Outfield seating on the Main Level includes seating down both baselines and wrapping into home run territory. Yankee Stadium is among the most homer-friendly parks out to right field. As such, sections 205, 206 and 207 get littered with home run balls. If you do choose to sit there, make sure to bring the shades and a hat during a day game -- the sun will be beating down on you all day.
For a slightly more comfortable afternoon, choose seating in Sections 233 or 234, which are less likely to be in the sun and are close to men's, women's and family restrooms.
If sitting in 210, 211, 228 or 229, be prepared to turn your body to watch the action on the infield -- the angle towards home plate is quite poor.
Each section has up to 23 rows, and Rows 15 and above will give you a pretty good chance of being protected from the sun (except in right field).