If an indifference curve shows quantities where a consumer's utility from consuming two goods are equal, then why are there infinitely many indifference curves? From the diagram, you can have Q1,Q2 on IC1, but then you'd have Q1,Q3 on the other indifference curve, wouldn't that mean the consumer would receive the same utility from consuming Q1 and Q2 as well as if he had consumed Q1 and Q3, i.e. for a given Q1, the consumption of Q2 gives the same amount of utility as Q3? Shouldn't this mean that the indifference curve should be unique?
Utility is constant for all points $(q_B,q_A)$ on an indifference curve. So there is a number $u_1$ such that $$ \forall (q_B,q_A) \in IC_1: \ U(q_B,q_A) = u_1. $$ Similarly there is a number $u_2$, such that $$ \forall (q_B,q_A) \in IC_2: \ U(q_B,q_A) = u_2. $$ If $IC_1 \neq IC_2$ then $u_1 \neq u_2$.