FBS offers quantity cannot be changed. You would need to upload a new FBS offer to reflect the desired quantity increase.
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?
Who handles customer service complaints and feedback and provides product support for Fulfilled by Souq products and orders?
How can I monitor numbers related to Fulfilled by Souq in the selling center?
What do I do when I receive a new Fulfilled by Souq order?