Here are some guidelines for estimating space requirements for masking operations. These estimates are based on a projected largest table size of 500GB. In making masking space estimates, assume a "worst-case scenario."
For in-place masking:
2 * 500GB for the mapping table (the mapping table stores both the original and the masked columns. Worse case is every column is to be masked).
2 * 500GB to accommodate both the original and the masked tables (both exist in the database at some point during the masking process).
2 * 500GB for temporary tablespace (needed for hash joins, sorts, and so forth).
Total space required for the worst case: 3TB.
For at-source masking:
2 * 500GB for the mapping table (as for in-place masking).
2 * 500GB for temporary tablespace (as for in-place masking).
Sufficient file system space to accommodate the dump file.
Total space required for the worst case: 2TB plus the necessary file system space.
In either case, Oracle recommends that you set the temp and undo tablespaces to auto extend.
You can specify a tablespace for mapping tables during script generation. If you do not specify a tablespace, the tables are created in the tablespace of the executing user. Note that space estimations are provided during script generation, with resource warnings as appropriate. There are some situations, for example when using the shuffle format, that do not require a mapping table. In these cases, updates to the original table happen in-line.