This is the AGGS admission info. The exam is VR (50 minutes), Maths (50 minutes) and NVR (2 x 10 minutes) and is set by GL Assessment (formerly called NFER). You can buy packs of sample papers through this website to see the type of questions and standard.
The cohort that sits the exam are largely self-selecting, so I don't think knowing how many take it is that useful. In order to pass you need a total score of 334 or greater (maximum 423). This is calculated by taking the raw scores for the three exams, giving each of them equal weight, and then standardising the scores to give three scores out of 141, the total of which gives the final score. If your daughter has done CATs/NFER tests at school already, this works in exactly the same way.
Assuming your daughter gets a qualifying score of 334 or over you then need to look at the order in which places are awarded at the school. AGGS has an 8 mile radius that it uses, measured from the school in a direct line to your front door (ie as the crow flies). After *looked-after* children, they first admit sisters of girls already at the school who live within the 8 mile radius, then girls within the 8 mile radius, then sisters of girls already at the school outside the radius and finally girls outside the radius.
Looking at your previous posts you say that you are about 6 miles in a direct line from AGGS. When you ring Admissions to ask about a visit ask them how far they have gone out in terms of radius for each of the last 3 years (ie 2011, 2010, 2009 intakes) as this could help you establish your chances of a place. Before 2009 the criteria were different, so not as relevant. If you find that they have only gone out as far as 5 miles, then this will obviously have an impact on chances.
Please note that score achieved only comes into play if, say, there was one place left at the school and two girls lived equally close. In that situation the girl with the higher mark would get in. Other than that, score means nothing.
AGGS admits 174 girls in each year, although in some years (eg current Y8, Y9, Y10) they admitted 203. The current Y7 is only 174, though, as is the soon to be new Y7, so I understand.)
In terms of practice, there are plenty of VR, and NVR practice books to go at. Be aware that some of the publishers cover far too many types of VR questions than are actually used in the exams themselves. This link is one to a thread which gives excellent advice as to what order the practice material for VR should be attempted. viewtopic.php?f=12&t=4782
Make sure that you tackle Maths. The exam tests all KS2 material even though it takes place in the September of Y6. Look at the GL papers and you will see that you may need to cover stuff like algebra, probability and volume of shapes, in particular, along with general problem solving techniques.
It is impossible to say what % you need to be aiming at in the practice papers. You could get 50% in the actual exam, yet if everyone else gets a lower mark still end up 141 as a standardised score. That said, based on my daughters who are both at AGGS, I think an average of 85% is a good target.