How do I limit the maximum children for spamd? For if there is a large spam attack - too many processes are started and the system can run out of resources.

In Plesk 7.5 and 8.0 you can limit the number of simultaneously running SpamAssassin processes with 'SPAMASSASSIN_MAX_CHILDREN' option in /etc/psa/psa.conf:

SPAMASSASSIN_MAX_CHILDREN 5

You can specify a desired value then restart psa-spamassassin.
If the line is omitted then the default value is 20 for SpamAssassin 2.6 and 5 for SpamAssassin 3.x.

Since Plesk 8.1 SPAMASSASSIN_MAX_CHILDREN option is obsoleted but this value can be managed through Plesk CP. Use "The maximum number of worker spamd processes to run (1-5)" on the Settings tab of Server>SpamFilter page. The value is stored in 'misc' table in'psa' database:

# mysql -uadmin -p`cat /etc/psa/.psa.shadow ` psa -e "select * from misc where param='spamfilter_max_children'"
+-------------------------+-----+
| param | val |
+-------------------------+-----+
| spamfilter_max_children | 5 |
+-------------------------+-----+