Welcome to the Atlassian Developer Community, @EdShober!
To get the count of issues under the toDo, inProgress, and done states, given that you already have the version ID, you can use the Get version REST API and expand issuesstatus. The endpoint will be /rest/api/2/version/{versionId}?expand=issuesstatus and the result will look like this:
{
"self": "https://mysite.atlassian.net/rest/api/2/version/10001",
"id": "10001",
"name": "1.1",
"archived": false,
"released": false,
"projectId": 10000,
"issuesStatusForFixVersion": {
"unmapped": 0,
"toDo": 0,
"inProgress": 1,
"done": 1
}
}
Hope this helps.
Ian