SSOM Multi Thread Execution C# SharePoint

I saw many articles which says microsoft.sharepoint.dll is not thread safe.
But when I run below code via 10 to 20 parallel threads it doesn't throw any error.
Although when I tried to multi thread foreach loop within SingleThread() which is defined below. It start throwing error at "SPListItemCollection" level.

//Main method:
ParallelOptions myOptons = new ParallelOptions { MaxDegreeOfParallelism = 10 };
            int[] loop = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
            Parallel.ForEach(loop, myOptons, myloop =>
             {
                 SingleThread();
             });

//Other method.

public void SingleThread()
        {
            DateTime startdate = DateTime.Now;
            try
            {

            int count = 0;
            SPWebService service = farm.Services.GetValue<SPWebService>("");
            SPWebServiceCollection webServices = new SPWebServiceCollection(farm);
            foreach (SPWebService webService in webServices) //Traverse Web Services
            {
                foreach (SPWebApplication webApp in webService.WebApplications)
                {
                    string webApplicationUrl = webApp.GetResponseUri(SPUrlZone.Default).ToString();
                    SPSecurity.RunWithElevatedPrivileges(delegate ()
                    {
                        SPWebApplication webApplication = SPWebApplication.Lookup(new Uri(webApplicationUrl));
                        foreach (SPSite sps in webApplication.Sites)
                        {

                            if (sps != null)
                            {
                                string spSiteUrl = sps.RootWeb.Url;
                                //Console.WriteLine(spSiteUrl);
                                SPWeb oWebsite = (new SPSite(spSiteUrl).OpenWeb());
                                SPWebCollection collWebsite = oWebsite.Webs;
                                foreach (SPWeb web in collWebsite)
                                {
                                    if (web != null)
                                    {

                                        foreach (SPList list in web.Lists)
                                        {
                                            if (list.BaseTemplate.ToString() == "DocumentLibrary")
                                            {
//Error in below line when I tried to multi thread SPListItemCollection.
                                                    SPListItemCollection lstitemcoll = list.GetItems();
                                                    foreach (SPListItem lstitem in lstitemcoll)
                                                    {                                                        
                                                        count++;                                                        
                                                    }
                                                }
                                            }


                                    }
                                }
                            }
                        }
                    });

                }
            }
            DateTime enddate = DateTime.Now;

            Console.WriteLine("Single Thread Count : {0}. Completed in {1} Seconds", count, (enddate - startdate).TotalSeconds);
        }
        catch (Exception ex)
        {
            string message = ex.Message;
        }
    }