My suggestion/experience:
bb (Aug-Dec), as many as you can
bb (Jan-Jul), 5-10 are okay.
Remember, during interview, your child is not USUAL, illness, being late, moody, hungry, sleepy, unattractive teachers, crowded environment, etc are all elements affecting your child's performance.
If any school accept my son, I think I will pay for the fee first to reserve a seat(to play safe). But at the same time I will still wait for the one which I like the most. My hubby said maximum let me pay to 3 schools @.@