There are a number of threads on the 13+ but you have to find them one by one as the search facility fails!
Preparation, use the CGP books for KS3 Maths, English and Science to fill in the gaps.
IIRC this year 76 sat for the 12 places. Lots of those sitting will be at indie schools where they have to move at age thirteen.
Entering Reading in Y9 is a challenge DS1 in Y12 feels that about half struggled and a few left after GCSEs. But if your DS is up for it, go for it.
My DS2 expressed an interest
in taking this years 13+ but we decided for him Reading might be an option at sixth form, he is still lacking in focus and application. This year the sixth form took in about 40 boys, points system equates to 2As and 6Bs at GCSE. The school are keen to take as many as they can accommodate for funding reasons!