@futureのサンプル

AccountProcessor.cls

public class AccountProcessor {
    
    @future
    public static void countContacts(List<Id> accountIds) {
        List<Account> accounts = [SELECT Id,(SELECT Id FROM Contacts) FROM Account WHERE Id IN: accountIds];
        for (Account a : accounts) {
            a.Number_of_Contacts__c = a.Contacts.size();
        }
        if (!accounts.isEmpty()) {
            update accounts;
        }
    }
}

AccountProcessorTest.cls

@isTest
private class AccountProcessorTest {

    private static User testAdminUser = new User(Id = UserInfo.getUserId());

    static testMethod void countContactsTest() {
        
        System.runAs(testAdminUser) {
            
            Account account = new Account(Name = 'Sample');
            insert account;
            
            Contact contact = new Contact(LastName = 'Yoshikawa', AccountId = account.Id);
            insert contact;
            
            List<Id> accountIds = new List<Id>();
            accountIds.add(account.Id);

            Test.startTest();
            
            AccountProcessor.countContacts(accountIds);
            
            Test.stopTest();
            
            Account resultAccount = [SELECT Number_of_Contacts__c FROM Account WHERE Id =: account.Id LIMIT 1];
            System.assertEquals(resultAccount.Number_of_Contacts__c, 1);
        }
    }
}