THE CODE USED

trigger  NumberOfContacts on Contact (after insert) {
set<id> setid = new set<id>();

list<account> listaccount = new list<account>();
for(contact con : trigger.new){

setid.add(con.accountid);
}

For(account acc : [select id,name,Number_of_Contacts__c, (select id from contacts)from account where id=:setid]) {
Account ac1 = new account();

ac1.id=acc.id;

ac1.Number_of_Contacts__c= acc.contacts.size();

listaccount.add(ac1);
}

The ITEST CODE

@istest
02public class testnumberofcontacts {
03    @istest static void checknumbers(){
04        test.starttest();
05        account testaccount = new account(name = 'Test account');
06        insert testaccount;
07        contact testcontact = new contact(lastname = 'Samplename', accountid = testaccount.Id);
08        insert testcontact;
09        test.stopTest();
10        list<account> queriedaccount = [select name, number_of_contacts__C from account where id =: testaccount.Id];
11        decimal resultcount;
12        for(account a: queriedaccount){
13            resultcount = a.Number_of_contacts__c;
14        }
15        system.assertEquals(1, resultcount);
16    }
17 
18}

THE 7! FAILURES

CMBatchCampaignStatsTesttest_LinkedCampaignsSystem.AssertException: Assertion Failed: Expected: 25, Actual: null 
Stack Trace: Class.wbsendit.CMBatchCampaignStatsTest.test_LinkedCampaigns: line 58, column 1
CMBatchCampaignStatsTesttest_LinkedCampaigns_doRun_UpdateOnSystem.AssertException: Assertion Failed: Expected: 25, Actual: null 
Stack Trace: Class.wbsendit.CMBatchCampaignStatsTest.test_LinkedCampaigns_doRun_UpdateOn: line 93, column 1
CMBatchCampaignTesttest_addMissingMemberStatusesSystem.AssertException: Assertion Failed: Expected: 3, Actual: 1 
Stack Trace: Class.wbsendit.CMBatchCampaignTest.test_addMissingMemberStatuses: line 212, column 1
DisconnectControllerTesttest_Savewbsendit.fflib_SecurityUtils.CrudException: You do not have permission to update Campaign. See: http://support.beaufort12.com/customer/en/portal/articles/2819881 
Stack Trace: Class.wbsendit.fflib_SecurityUtils.checkObjectIsUpdateable: line 413, column 1 Class.wbsendit.fflib_SecurityUtils: line 272, column 1 Class.wbsendit.DisconnectController.disconnect: line 26, column 1 Class.wbsendit.DisconnectControllerTest.test_Save: line 32, column 1
LinkCampaignControllerTesttest_autoCreateSalesforceCampaignswbsendit.fflib_SecurityUtils.CrudException: You do not have permission to insert Campaign. See: http://support.beaufort12.com/customer/en/portal/articles/2819881 
Stack Trace: Class.wbsendit.fflib_SecurityUtils.checkObjectIsInsertable: line 388, column 1 Class.wbsendit.fflib_SecurityUtils: line 137, column 1 Class.wbsendit.LinkCampaignController.autoCreateSalesforceCampaigns: line 191, column 1 Class.wbsendit.LinkCampaignControllerTest.test_autoCreateSalesforceCampaigns: line 78, column 1
LinkCampaignControllerTesttest_SaveSystem.AssertException: Assertion Failed: Same value: null 
Stack Trace: Class.wbsendit.LinkCampaignControllerTest.test_Save: line 38, column 1
ReportToListBatchTesttest_doReportToBatchSystem.LimitException: reports:Too many query rows: 50001 
Stack Trace: External entry point Class.wbsendit.ReportToListBatch.getNumberOfRows: line 469, column 1 Class.wbsendit.ReportToListBatch.doStart: line 142, column 1 Class.wbsendit.ReportToListBatch.start: line 48, column 1