limit the number of samples in a preparation

Add a new parameter in the database to limit the number of samples allowed in a single preparation. 

The current number of samples per lane within the KL are:
- Generally we run 768 per lane for metagenomics but 1536 per lane may happen soon
- 16S the max we can sequence per run is 800 since we only have 800 unique barcodes

Thus, decided to go with 800.