There is no minimum amount of resin that can be mixed however there is a risk/challenge with the mixing ratios. When you mix the Alumilite Amazing Clear Cast it is at a ratio of 1:1, if these measurements are not exact with larger quantities it is unlikely to cause any issues, however if you are mixing a small amount even the slightest difference in the measurements could cause an issue as this will be a bigger percentage of the mixture. So with a smaller quantity it is essential to make sure the measurement of part A and B are exact.
With a 2:1 resin such as Alumilite Amazing Deep Pour the ratios can be even harder to achieve and are much more effected by incorrect volumes so doing a small pour of this particular resin could be difficult.
The other challenge you may face is that there will be less volume of resin and therefore there is less to trigger a reaction, you will need to be careful to not work in a cold room or pour into a cold mould as with small quantities of resin it is more likely the heat would be pulled out meaning it would not cure.