FBS offers quantity cannot be changed. You would need to upload a new FBS offer to reflect the desired quantity increase.
Operational & Safety Guidelines at Souq Fulfillment Centers
Souq Fulfillment Centers : Locations and Contact information
What is Fulfilled by Souq (FBS)?
Listing and Creating FBS Shipping Notices
How can I register with Fulfilled By Souq?
Can I request to recall my Fulfilled by Souq products from your fulfillment centers?