If you have APs, it counts towards your transferable units. Anyway, I suggest you take at least 6 semester units in the summer (it would be best if you can take 9). Depending on how many APs you took and what you got on it, you should take at least 18 during the spring. Well, I really can't tell you how many units to take each semester. But if you want to transfer in 1 year, you can't take "one more summer school" because you have to finish 60 units by SPRING.
Btw, from personal experience, don't take classes just because they have more unit value than others. For example, Mechanics (Physics) is usually 5 semester units. However, you are required to attend 72 hours of lecture and 72 hours of lab. On the other hand, if you were to take two 3 unit classes, it would only be 108 hours together. So you do the math. You spend a lot less time at school if you take the two 3 unit class.
Am I making sense?