The queries identify the number of unique transactions and the number of lines in the table. Dividing one by the other will reveal the average lines per transaction.
This query reveals the Number of Inventory Transactions processed on a given day and the number of overall lines.
DECLARE @Date as DATETIME
SET @Date = '04/12/2017'
/* Inventory Transactions */
Select SUM(a.IV_Trxs) IV_Trxs from (
select count(I.IV_TRXS) IV_Trxs from (
select count(IVDOCNBR) IV_Trxs from IV10001 group by IVDOCNBR) I
select COUNT(IH.IV_TRXS) IV_Trxs from (
select COUNT(DOCNUMBR) IV_Trxs from IV30300 WHERE DOCDATE = @Date group by DOCNUMBR) IH ) a
/* Inventory Transaction Lines */
select SUM(IV_Trxs) from (
select COUNT(I.IVDOCNBR) IV_Trxs from IV10001 I
Select COUNT(DOCNUMBR) IV_Trxs from IV30300 where DOCDATE = @Date) a