Azure DevOps Repo(s) setup

Greetings.  I am in the process of implementing Azure DevOps (on Prem) at my company and have a few best practices question.

Small bit of Background:

We sell a packaged product that consists of a handful of VS2019 projects (4 winform applications, 1 windows service, 1 rest api, and a handful of wix installer projects).  We do quarterly releases and distribute our product in the form of a handful of .msi's.  All of the development is done inhouse by 5 developers.  We plan on using the following Git branching model: https://nvie.com/posts/a-successful-git-branching-model.  All  VS2019 projects are versioned and released together.

Azure Git Repo Question:

1. Would it make sense to have just a single repo to house the source code for all of the VS2019 applications?  This way I would only have to set one set of branching policies and permissions.  It also saves time on repo setup and maintenance.  Is this advisable?  Is there a reason why multiple repos are a better option?
2. If the use of a single repo is advisable, how would we handle CI builds and builds when Pull Requests get submitted?  If the pull request is for code in only one application, I don't want to build the other ones.

Thanks in advance,

Scott