List<Account> accounts = [SELECT Id, Account_Number__c FROM Account ORDER BY CreatedDate ASC];
        List<Account> accountsUpdate = new List<Account>();
        Integer HOW_MANY_OBJECTS_AT_ONCE = 1100;
        Integer counter, endCounter;
        String templateOfField = '';
        Integer indexOfNOTFilledAccountNumberField = 0;
        for (Account account : accounts) {
            if (String.isBlank(account.Account_Number__c)) {
                indexOfNOTFilledAccountNumberField = accounts.indexOf(account);
                break;
            }
        }
        counter = indexOfNOTFilledAccountNumberField;
        endCounter = indexOfNOTFilledAccountNumberField + HOW_MANY_OBJECTS_AT_ONCE - 1;

        for (; counter < accounts.size(); counter++) {
            templateOfField = 'A-';

            if(counter < 1000) {
                if (counter < 10) {
                    templateOfField += '000';
                } else if (counter < 100) {
                    templateOfField += '00';
                } else {
                    templateOfField += '0';
                }
            }

            if (String.isBlank(accounts.get(counter).Account_Number__c)) {
                accounts.get(counter).Account_Number__c = templateOfField + String.valueOf(counter);
                accountsUpdate.add(accounts.get(counter));
            }

            if (accountsUpdate.size() == endCounter) {
				break;
	    	}
        }

        System.debug('accounts.size() = ' + accounts.size());

        if (accountsUpdate.size() > 0) {
            System.debug('indexOfNOTFilledAccountNumberField = ' + indexOfNOTFilledAccountNumberField);
            System.debug('accountsUpdate.size() = ' + accountsUpdate.size());
            update accountsUpdate;
        } else {
            System.debug('*****UPDATE FINISHED*****');
        }