Correct Random Number Generator to create only 15 or 16 digit number in Data Masking

mysql> SELECT mask_pan(gen_rnd_pan());
+-------------------------+
| mask_pan(gen_rnd_pan()) |
+-------------------------+
| XXXXXXXXXXXX7871        |
+-------------------------+
1 row in set (0.00 sec)mysql> SELECT mask_pan(gen_rnd_pan());
+-------------------------+
| mask_pan(gen_rnd_pan()) |
+-------------------------+
| XXXXXXXXXXXX4808        |
+-------------------------+
1 row in set (0.00 sec)mysql> SELECT mask_pan(gen_rnd_pan());
+-------------------------+
| mask_pan(gen_rnd_pan()) |
+-------------------------+
| 60111451419726510       |
+-------------------------+
1 row in set (0.00 sec)mysql> SELECT mask_pan(gen_rnd_pan());
+-------------------------+
| mask_pan(gen_rnd_pan()) |
+-------------------------+
| XXXXXXXXXXXX2936        |
+-------------------------+
1 row in set (0.00 sec)