Classic asp recordcount not working

Retrieve recordcount property from an ADO recorset give everytime -1, i tried all combination of cursortype and locktype, nothing changed. Tried also use movelast and movefirst as suggested on forums in the web but the problem persists.
Run this code to reproduce the issue
<%
Set Conn = Server.CreateObject("ADODB.Connection")
Conn.Open("DRIVER=

;port=3306;SERVER=127.0.0.1;UID=XXX;PWD=XXX;database=test;charset=UTF8;")
SQL = "SELECT codArch FROM archivi;"

set RS = Server.CreateObject("ADODB.RecordSet")
RS.open SQL,conn, 1, 3
'RS.movelast
'RS.movefirst
response.Write "record: "& RS.recordCount

RS.close
set RS = Nothing
Conn.close
set Conn = Nothing
%>

the table structure is
CREATE TABLE IF NOT EXISTS `archivi` (
`codarch` int(11) NOT NULL DEFAULT 0,
`codcli` int(11) DEFAULT NULL,
`descrizione` char(50) DEFAULT NULL,
`perc` char(255) DEFAULT NULL,
`mariadb` char(50) DEFAULT NULL,
`codtipoarch` int(11) DEFAULT NULL,
`Bloccato` int(11) DEFAULT NULL,
`controlloRichieste` int(11) DEFAULT NULL,
`dataCaricamentoDoc` char(50) DEFAULT NULL,
`ordine` int(11) DEFAULT NULL,
`visibile` int(11) DEFAULT NULL,
`codStatoArch` int(11) DEFAULT NULL,
`dataRitiro` datetime DEFAULT NULL,
`dataMacero` datetime DEFAULT NULL,
PRIMARY KEY (`codarch`),
KEY `codarch` (`codarch`),
KEY `codtipoarch` (`codtipoarch`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci;