You can "read" the number of strings but can't change the predefined number of strings without creating a new TEXT (txt_create(...))
To read the number of predefines strings use: [TEXT].strings (where [TEXT] is the TEXT-Object)
To read the used number of strings you have to loop over all strings and get the last with str_len > 0
To change the number of strings you have to create a new TEXT-object, copy the content of the "old" to the "new" and remove the "old" TEXT-object.
mercuryus