sa-learn --dump magic

returns:

0.000          0          3          0  non-token data: bayes db version
0.000          0        306          0  non-token data: nspam
0.000          0        210          0  non-token data: nham
0.000          0      68430          0  non-token data: ntokens
0.000          0 1318421928          0  non-token data: oldest atime
0.000          0 1319141693          0  non-token data: newest atime
0.000          0 1319142287          0  non-token data: last journal sync atime
0.000          0 1319142287          0  non-token data: last expiry atime
0.000          0          0          0  non-token data: last expire atime delta
0.000          0          0          0  non-token data: last expire reduction count

nspam and nham indicative the actual amount of learning and messages that SpamAssassin is using for its Bayesian analysis.

You can find out the current numbers by:

sa-learn --backup

The first 3 rows show you the number of Spams (num_spam) and Ham (num_nospam).