Have managed to put more than 100k urls :
Update the below function in :
Nop.Services\Seo\SitemapGenerator.cs
protected virtual IEnumerable<SitemapUrl> GetCustomUrls()
{
var storeLocation = _webHelper.GetStoreLocation();
var customUrls = new List<string>();
var filePath = Path.Combine(_hostingEnvironment.ContentRootPath, "App_Data", "custom-urls.txt");
if (File.Exists(filePath))
{
customUrls = File.ReadAllLines(filePath).ToList();
}
var sitemapUrls = new List<SitemapUrl>();
foreach (var customUrl in customUrls)
{
var url = string.Concat(storeLocation, customUrl);
sitemapUrls.Add(new SitemapUrl(url, new List<string>(), UpdateFrequency.Weekly, DateTime.UtcNow));
}
return sitemapUrls;
//return _sitemapXmlSettings.SitemapCustomUrls.Select(customUrl =>
// new SitemapUrl(string.Concat(storeLocation, customUrl), new List<string>(), UpdateFrequency.Weekly, DateTime.UtcNow));
}