Defines the number of blend tables used by alpha.
The IDs for the alpha tables start from 1, because the first ID is reserved to 33% transparency.
The transparency of the alpha tables should range from 0% to 50%. The second half will be generated by reversing the first half.
For example, if you define numalphatables as 16, you should import 17 tables using blendtable, and in-game there will be 32 steps of transparency.
You can import up to 128 tables, which would provide a table for every value of alpha. However a value so high is probably not necessary, since the palette of a game won't have enough steps for a color to justify such a value. It would be more reasonably to use a value similar to the shade table, which in Duke 3D case is 32, so a value of 16 for numalphatables would be more logical.