Rob
Ansys Employee

With that time step you're adding 1000 parcels per inlet facet per injection, ie you add a parcel per facet per particle time step. That may result in several parcels in the domain: the solver will report this in the TUI. 

You're also going to struggle with a max steps of 5,000,000 when the default is 500 (from memory). If you need that many steps to avoid incomplete trajectories you need a smaller time step. If you do need that many steps the cpu requirement is going to be excessive.