You'll want to use a
List. Probably a
List<InventoryItem> or similar.
The documentation for
List<T> is here.
Here is a set of answers on how to serialize a list to XML (you didn't say how you were saving your XML - this link gives you some options).
An XML file can store the list of items "inline", so it might end up something like:
On the other hand, if you switched to a database (using SQL), the traditional way to store something like this would be to have player table (with a "key" value, like an ID number) and a inventory table for every item in the game - each having an "owner" value that matches the ID of the owning player.
You would then use SQL to query for all the items in the inventory table owned by a given player (ie: having the matching ID). Something like:
SELECT item_name,item_whatever FROM inventory WHERE owner_id = <whatever>
The correct solution probably depends on the scale of your game. But if you're unsure - go with the XML to begin with - it is easier to implement - you can alway change later if the need arises.