var report = await pdfService.RenderAsync(new RenderRequest
{     
     Template = new jsreport.Types.Template
     {
          Recipe = Recipe.ChromePdf,
          Engine = Engine.Handlebars,
          Name = "Violation Letter"
     },
     Data = data
});

The solution needs to employ the pdf utils now.
Here is the demo
https://playground.jsreport.net/w/anon/NjyiLidA

In c# without a stored template it would be something like this:

var customReport = rs.RenderAsync(new RenderRequest()
{
    Template = new Template()
    {
        Content = "Helo world",
        Engine = Engine.Handlebars,
        Recipe = Recipe.ChromePdf,
        Scripts = new List<Script>
    {
        new Script
        {
            Content = @"const jsreport = require('jsreport-proxy')
async function afterRender(req, res) {
    let $pdf = await jsreport.pdfUtils.parse(res.content)
    res.meta.headers.numberOfPages = $pdf.pages.length
}"
        }
    }
    }
}).Result;
Console.WriteLine(customReport.Meta.RawDictionary["numberofpages"]);