How you use your balance, how many balances you have and your Quality Management System will determine how many and what nominal value test weights you require.
If you only use the balance at a specific value you may only need a single weight to complete your routine monitoring and testing. However, if you have multiple balances, with multiple maximum capacities and use each one at differing values then you will need a range of test weights to complete your routine monitoring and testing.
Be aware though that although you can use higher accuracy weights for lower accuracy balances you cannot use lower accuracy weights for higher accuracy balances. The reason for this is that, even though the test weights are calibrated, you must take into account not only the stated error on the calibration certificate but also the uncertainty of that measurement. It is a parameter, associated with the result of a measurement (e.g. a calibration or test) that defines the range of the values that could reasonably be attributed to the measured quantity, such as environmental factors and air buoyancy and even gravity.