how to upload multiple excel files to sql table

First, I'd suggest using EPPlus for managing those Excel files. You can get it via NuGet. Add a reference to OfficeOpenXml in your using statements.

So you'll want to read in those files one at a time, get the values from each worksheet, add to a list of objects, and run an INSERT statement against your SQL database. There are many ways to do this, but this is quick and dirty using ADO.NET and dynamics. Code sample assumes you're using a console application and Microsoft.NET 4.6.

Code below is not tested, just an example of how you might do this.

private static void ProcessFiles()
{
    // Get a reference to your directory
    var directory = new DirectoryInfo(@"C:\MyFiles");

    // Get all Excel files
    foreach (var file in directory.GetFiles("*.xslx"))
    {
        // Create a new package instance from the given file
        using (var package = new ExcelPackage(file))
        {
            // Get a local reference to the worksheet
            var workSheet1 = package.Workbook.Worksheets[0];

            // Create a list to hold your objects
            var objectList = new List<dynamic>();

            // Loop over the rows
            for (var x = workSheet1.Dimension.Start.Row; x <= workSheet1.Dimension.End.Row; x++)
            {
                // This is the sample value of the entry on sheet 1, row x. You can 
                // use this to create a dynamic object to insert into your database

                dynamic objectToInsert = new ExpandoObject();
                objectToInsert.Value1 = workSheet1.Cells[x, 0].Value;
                objectToInsert.Value2 = workSheet1.Cells[x, 1].Value;
                // .. Continue as needed

                // Once you have all your values in your object, add it to the list
                objectList.Add(objectToInsert);
            }

            // Insert into your database
            using (var conn = new SqlConnection("MyConnectionString"))
            {
                foreach (var row in objectList)
                {
                    // Better idea is to use a stored procedure, but this will get the                     
                    // job done
                    using (var cmd = new SqlCommand($"INSERT INTO dbo.MyTable (Value1, Value2) VALUES ({row.Value1}, {row.Value2})", conn))
                    {
                        cmd.ExecuteNonQuery();
                    }
                }
            }

            // .. Repeat for the next worksheet
            var workSheet2 = package.Workbook.Worksheets[1];
        }
    }
}