Hi Eduardo, you're right. The spacer adquisition has a limit, basically restrcited by the size of bacterial genomes. But the mechanisms by which this occurs have not been identified. I know that internal deletions in regions near to trailer sequence in the CRISPR locus play an important role in the process. It deletions allow that  new spacer be adquired in the leader region. It review is really interesting: DOI: 10.1146/annurev-genet-110410-132430. I'm attaching the pdf paper.

Best,

Lisandro.