Q.How to find the maximum loading rate of the ship?

P&A Manual and check the drop calculation on VECS manual

Where a number of tanks are loaded through a common manifold, the maximum loading rate may be determined by the flow rate through the manifold/drop lines.  Therefore you need to do an additional exercise basis the vetting arrangements + flow rates in order to find what is the max loading rate per manifold, for homogenous cargo (loading in all tanks or min 4 tanks).

As per ISGOTT 7.3.3.2 :

....

* A loading rate based on a linear velocity of 12 metres/second for loading non-static accumulator cargoes and also for loading static accumulator cargoes into inerted tanks. This velocity is provided for guidance only and is generally considered as a rate above which pipeline erosion may occur at pipe joints and bends.

Considering that if yr vessel's size of manifold/drop lines is 16" or 406 mm as per the attached from ISGOTT yr ship can load 5600 m3 per hour with 1 manifold. Basis loading in 4 tanks the ship can declare max loading rate homogenous cargo for 1 manifold 5333 m3/hr.