The way CAL licensing works, you need one for every application and user that accesses the database. So if you are just using Backup & Replication - 1 for B&R and 1 for each user that accesses the console.
If you are using VeeamONE and Enterprise Manager you would probably need one each for those to not run afoul of MS.
Since SQL CALs are, iirc, for concurrent users you could probably get away with the application ones plus what you think would be the average number of concurrent users and be ok.
Veeam Certified Architect