Trigger:
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trigger AccountTrigger on Account (before insert) {
    if(Trigger.isBefore) {
        if(Trigger.isInsert) {
            AccountTriggerHandler.onBeforeInsert(Trigger.New);
        }
    }
}

Handler:
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public class AccountTriggerHandler {
    public static void onBeforeInsert(List<Account> accounts) {
        
        Integer concatinatedValue     = 1234567890;
        Integer accountSize         = accounts.size();
        
        // To get the Last Inserted Record with 10 digits
        Account account = getLatestAccount();
        
        // condition to check for the record
        if(account != null) {
            // To get the 10 digit number from Name
            Integer intValue = getIntFromName(account.Name);
            
            // Condition to check if the 10 digit exists or not
            if(intValue != null) {
                concatinatedValue = intValue;
            } else {
                concatinatedValue = concatinatedValue - 1;
            }
        }
        
        // Condition to check the record count
        if(accountSize > 0) {
            // Loop to iterate over the List of records
            for(Integer i=0; i < accountSize; i++) {
                concatinatedValue = concatinatedValue + 1;
                accounts[i].Name = accounts[i].Name + concatinatedValue;
            }
        }
    }
    
    // Method to get the Last inserted Record based on created Date for the existing records
    private static Account getLatestAccount() {
        
       List<Account> accounts = [SELECT ID, NAME FROM Account ORDER BY CreatedDate Desc];
        if(accounts.size() > 0) {
            return accounts[0];
        }
        return null;
    }
    
    // Method to spilt the first 4 characters from concatinated number
    private static Integer getIntFromName(String Name) {
        String phrase = Name;
        Integer rephrase = null;
        if(phrase != null && phrase.length() > 4) {
            phrase = phrase.substring(4, phrase.length());
            if(phrase.isNumeric()) {
                rephrase = Integer.valueOf(phrase);
            }
        }
        return rephrase;
    }
}

Test class:
------------------
@isTest
public class AccountTriggerHandlerTest { 

    @isTest static void AccountTest() {
        Account account = new Account(Name = 'WXYZ');
        insert account;
        
        List<Account> accounts = new List<Account>();
        for(Integer i = 0; i < 10; i++) {
            Account account1 = new Account(Name = 'ABC' + i);
            accounts.add(account1);
        }
        insert accounts;
        
        System.assert(accounts.size() > 0);
    }
}