According to the Bucks CC websit you get six choices. From their timeline you find out whther they've passed or failed the 11+ before you have to make the application.
www.buckscc.gov.uk/services/education/school-admissions/moving-up-to-secondary-school/timeline-for-entry-in-2019/
I had always understood that if you passed the 11+ you were guaranteed a grammar school offer but it appears this may not be the case (see below)so perhaps wise to put a secondary modern on the form just in case? Perhaps someone who knows the Bucks system could confirm?
If we can’t offer any of your preferences, we try to offer a place at the nearest Buckinghamshire school. This may not be your catchment school. If your child is qualified for grammar school, we will try to offer a place at the nearest Buckinghamshire grammar school. However, in recent years this has not been possible as all grammar schools have been full on 1 March. This is why it is sensible to not just include Buckinghamshire grammar schools on your application.
Also, if your child fails the test you can still apply to a grammar school if you are intending to appeal:
*If your child does not qualify for a grammar school, but your preferred school is still a grammar school, then in order to continue pursuing this option, you must include a grammar school preference on your application. You may then request a Selection Review or, from 1 March 2019 (National Offer Day) onwards, an admission appeal. When completing your application, it is best to put any grammar school preference above any upper/all-ability school preference.
At the time secondary school places are allocated on 1 March 2019, if your child has not qualified for a grammar school and you have included grammar school preferences on your application, we will automatically refuse them. However, you will be entitled to appeal this refusal. If you have not included a grammar school preference on your application then you will not be able to appeal for that school, as it is the refusal of this school allocation on 1 March that triggers the right of appeal.*
Hope this all makes sense!