How much free space on the fileserver is needed to allocate the CIFS that senhasegura will use for backups

There is no prior way to do it. It is necessary to evaluate how much disk space will be consumed daily (or monthly) and then, based on the purge time, scale.
Bearing in mind that, so that several resizing is not necessary, the ideal is to carry out the projection taking into account an extra space for safety and an increase in the use of the tool must also be estimated.

Ex: If today an average of 3Gb is used per day and the prediction is that the vault will double in user in 3 months. The calculation must account that in 3 months the daily average will be 6Gb.