I will do my best to explain! With your PicoScope 3425 the waveform buffer is created in main PC memory and its about 100M bytes. As the 3425 is a 12 bit oscilloscope that means that you can store around 67,000,000 samples in the buffer.
If you set the buffer to 1000 waveforms then that means you can collect 1000 waveforms, each of 67,000 samples.
If you allow the scope to collect more samples per waveform (the max for your product is 512,000) then the maximum no of waveforms that can be stored in the buffer is reduced. In the example below you collect 858 waveforms each of 78,130 samples and if you do the sums thats 67,035,540.
The reason the number of samples per waveform varies is due to the timebase. At faster timebases there is not enough time for the scope to collect the max no of waveforms as indicated at the top of the screen.
If you want 1000 waveforms then set the max no of samples to 67,000 or less then you should never run out of memory. If you want you could actually set the max no of waveforms to > 1000 then at timebases that limit the no of samples to less that 67,000 then you will be able to have more than 1000 waveforms.
Note that the above is specific to to products with less than 1M of internal memory. For products with deeper memory the waveform buffer is created in the oscilloscope hardware rather than PC memory as this reduces the load on the PC.