You are posting from China, not USA. Kindly use ISO units to give the correct international flavour. Incidentally the title quotes 2700 and the data quotes 2370 of some 'old school' units.

But, even so, it is clear that your system is awry. Normal pumping service employs a 2+1 arrangement. If the job had been done properly from day 1 the motor size could be reduced along with the pump size. Casing strain would be relieved along with the maintenance loophole. If 2 of 3 go down you can still limp along. If 2 out of 2 go down you are seriously embarrassed.

As for the rest; look at the some manufacturers data. It is quite straightforward: you have to deliver whatever with something in reserve, as usually happens.

You haven't described the existing pump construction apart from the slurry duty. Regardless of your spares stockholding I would still recommend 3 downsized WATER pumps. An 8.1MPa delivery head will take some starting and develop a severe casing pressure in the latter pump stages. ■