There's two factors that matter with MDS/MDM licensing:
- Number of Domains (or CMAs)
- Total number of gateways managed across all domains
Domain licensing (CPSB-DMN-X) is additive, as I recall, though you're usually better off trading up for the next largest one.
Or, in your case, buying a CPSB-DMN-50.
Total number of gateways managed is not except in the case of 150 gateways, e.g. CPSM-NGSM150 where you can add chunks of 50 gateways using CPSB-GW-EXT-50.
The above is in terms of Open Server licenses.
If you're buying Check Point Appliances, you're most likely in the Smart-1 6000 range with additional licenses.