It depends on what backend you're using, but I'm not sure that's possible. When an email is learned the tokens are taken from it and stored in the database and a single email could contain multiple tokens. I don't think it ever records how many emails it's learned from because that's a rather useless statistic.
If I look at my database I get 2755 and I don't think I've fed sa-learn 2755 messages yet.
mysql> select count(id) from bayes_token;
+-----------+
| count(id) |
+-----------+
| 2755 |
+-----------+
1 row in set (0.00 sec)