My thought is to ask them, casually, how they arrived at 10 hours (without giving any inkling that you disagree with it). Let them tell you.
Then ask which hours the support will be provided for. Is it 2 hours every morning, or 5 hours twice a week?
And then, what is the supporting person going to do? (Is it keep DD on task, and help her understand the teacher?)
The 10 hours is probably worked out on a matrix of funding with the kids they deem most severe at one end of the matrix, and the least severe at the other.
Whatever their answers to the question, it will probably make no sense whatsoever when you consider that your DD has ASD 24/7. If she needs adult support to keep her on task, then are they saying that her ASD disappears for all the other hours in the day?
It is just possible that they have noticed her problems are at break time and lunch time and that she is fine in the classroom. Or that her concentration lapses only for the last two hours of the day. So they have worked out that this is the only time she needs support, ie that there is a rational explanation worked out on a careful analysis of your child's needs. I will be completely gobsmacked if that IS the case, but it is a logical possibility, just very unlikely in my experience.
Your DD is entitled to full time education that meets all her needs. If she needs support to engage appropriately in lessons, then they cannot get away with giving it for only a few lessons.
AND with EHCP you have 'best possible outcome' as the goal, so partial engagement in lessons is scarcely likely to be that.
Hope this is hitting the spot