Cell Counter Validation

Hello ,
I have to validate a Cell counter or analyzer, which is a cell suspension analysis system. Like all instruments nowadays it has the actual instrument and then a PC with operating software. My question is how much functional testing should I do as part of my OQ.

The software has many capabilities but if i start testing them all, it will take me a lot of time. On the downside, I dont want to do a getting away with easy kind of validation too