Ah, OK, I thought I'd read somewhere that if you reached the limit, it just did not do any further backups.
[edit]
Just checking and the max number is a per directory setting. Unless you create a new directory for each container, if you do not backup everything, then you could lose the backup of a specific container.
For instance, I have a container for mosquito Broker - there is no point regularly backing this up, it just needs a backup if the configuration is changed. A backup for the node-red container needs backing up regularly as the configuration changes regularly.