A Year 6 maths group from Devonshire Primary School sent us a solution for this problem:

The 10am coach will see the 8.40, 9.00, 9.20, 9.40, 10, 10.20, 10.40, 11, 11.20 coaches on its way to Betaville. Therefore it sees 9 coaches. We know this because they are travelling at the same speed and on the same path.

Paul, aged 9, who doesn't say where he's from, also agreed with this answer. Thank you to you all.