Channels and PoE ports
Channel count is the number of supported camera streams, subject to recorder limits. Built-in PoE port count is a separate specification. Also check incoming bandwidth and decoding capacity.
How long will footage last?
At a constant 4 Mbps per camera, six cameras produce about 259 GB per day. A nominal 4 TB disk gives roughly 15 days before formatting, overhead and reserve space. Actual bitrate and usable capacity change the result.
Continuous or event recording?
Continuous recording provides a full timeline. Event recording can save space but depends on suitable triggering and settings. Neither guarantees a fixed retention period without a storage calculation.
Exports and overwrite
Most systems can overwrite old recordings when configured to do so. Export important clips promptly and test that they play on another device. RAID, where supported, is not a substitute for a separate copy of critical evidence.