In general, you have to adjust the heat and airflow to batch size. Use less heat and fan for smaller batches.
But how little can you roast in a given roaster? It depends on how much you can turn down the heat. And then you have to be aware of what it take to get a fair reading with the Bean temperature probe. To tell what is going on in the beans the probe has to be covered in beans.
Aillio says minimum batch is 350 grams to have the right BT measurement on the Bullet R1.
With smaller batches the BT probe will not be sufficiently covered. But you can still roast smaller batches. Just ignore the BT reading.
Klaus investigated this with 100 grams batches to do sample roasts. Read about his settings here.
Diego Cano in Colombia uses the Bullet roaster to do a lot of sample roasting. He found that it works the best with 250 grams samples.