How to obtain the number of rows of data in a table tag through C # code ?

razor is a template engine, not a component model. its just a string. once rendered in the browser, you can use jsinterop to access the dom (assuming you are coding in blazor).

if you want access the table row data from c#, then change the template to use an object array:

@code {  
  var data = new[]
  {
    new {
        FirstName = "Lars",
        LastName = "Vileid",
        Product = "Caffe Espresso",
        Quantity = "Low",
        Price = "Low"
    },
    new {
        FirstName = "Petra",
        LastName = "Davolio",
        Product = "Caffe Latte",
        Quantity = 10,
        Price = "High"
    },
  };
}
<table>
    <thead>
        <tr>
            <th scope="col">First Name</th>
            <th scope="col">Last Name</th>
            <th scope="col">Product Name</th>
            <th scope="col">Quantity</th>
            <th scope="col">Price</th>
        </tr>
    </thead>
    <tbody>
    @foreach(var row in data) {
        <tr>
            <td>@FirstName</td>
            <td>@LastName</td>
            <td>@Product</td>
            <td>@Quantity</td>
            <td>@Price</td>
        </tr>
    }
    </tbody>
</table>