Type FT_Result = Integer;
var
	FT_Device_Count : DWord;
Const 
    FT_LIST_NUMBER_ONLY = $ 80000000 ;
    FT_DLL_Name = 'FTD2XX.DLL';
	FT_OK = 0 ;
implementation	
function FT_GetNumDevices(pvArg1:Pointer; pvArg2:Pointer; dwFlags:Dword):FT_Result; stdcall; External FT_DLL_Name name 'FT_ListDevices';

Собственно вызов
...
Result := FT_GetNumDevices(@FT_Device_Count,Nil,FT_LIST_NUMBER_ONLY);
...

DevNum= 0 
ParNull=NULL
#DEFINE FT_LIST_NUMBER_ONLY 0x80000000
DECLARE long FT_ListDevices IN FTD2XX.DLL long @Par1,long @Par2,integer Par3
?FT_ListDevices(@DevNum,null,FT_LIST_NUMBER_ONLY)
?FT_ListDevices(@DevNum,@ParNull,FT_LIST_NUMBER_ONLY)

The examples that follow use these variables.
FT_STATUS ftStatus;
DWORD numDevs;
 1 . Get the number of devices currently connected
ftStatus = FT_ListDevices(&numDevs,NULL,FT_LIST_NUMBER_ONLY);
if (ftStatus == FT_OK)
{ // FT_ListDevices OK, number of devices connected is in numDevs } else { // FT_ListDevices failed }