Hi Monnedepraetere,
If it will always be the first item of each array, you could use the | first filter and assign to ensure you only get one value.
Something like:
{% assign firstFulfillmentOrder = getOrderDataForeachitem.fulfillmentOrders | first %}
{% assign firstFulfillment = firstFulfillmentOrder.fulfillments | first %}
{% assign firstTrackingInfo = firstFulfillment.trackingInfo | first %}
{{ firstTrackingInfo.number }}