I have a simple question. According to this retention policy when doing more that 3-4 backups per day to a VM how many block level backups should be kept?
Since block-level backups rely on full backups to perform purge you will keep all of those block-level backups performed this day. The next time you run a full backup the software will detect all copies that are older than 1 day and purge them.
One thing to keep in mind: all of the data(FULL + Block-level) need to be older than one day, since FULLs can't be purged separately from DIFFs and vice-versa.