Your first Xpath Expression for Item/ItemNumber is wrong

Instead of 
N.value('(Item/ItemNumber[1])','int')
You want 
N.value('(Item/ItemNumber)[1]','int')


declare @packXML XML = '<Pack>
<PackID>1</PackID>
<Item>
<ItemNumber>1</ItemNumber>
<Qty>1</Qty>
</Item>
</Pack>
<Pack>
<PackID>2</PackID>
<Item>
<ItemNumber>1</ItemNumber>
<Qty>5</Qty>
</Item>
<Item>
<ItemNumber>2</ItemNumber>
<Qty>10</Qty>
</Item>
</Pack>
'
select N.value('PackID[1]','int'), N.value('(Item/ItemNumber)[1]','int'), N.value('(Item/Qty)[1]','int')
from @packXML.nodes('Pack') as T(N)