Hi Paavo,
please read https://developer.steinberg.help/display/VST/VST+3+API+Documentation
especially the paragraph IAudioProcessor where it states:
Block Size: Processing is done in blocks. The maximum number of samples to be processed in one block is set in Steinberg::Vst::IAudioProcessor::setupProcessing. The actual number of samples in a processing block is transmitted in the process call and can be different from call to call, but it must be a value between 1 and maxSamplesPerBlock.
Cheers,
Arne