At the moment the UI lets you assign a maximum of 5 soft credits for a contribution. Once you have reached that limit, the "another soft credit" button is no longer visible.
I am not sure how this number was decided on (No mention of a limit to the number of soft credits in http://wiki.civicrm.org/confluence/display/CRM/Soft+Credit+Enhancements+-+4.4 or https://issues.civicrm.org/jira/browse/CRM-12463.) but for some of us it is too low. http://forum.civicrm.org/index.php/topic,31797 .
It seems sensible to me to make a change to core to increase the limit on the number of soft credits. (We have "hacked core" to get around the limit for now.)
I can create the (trivial) PR to increase the limit to whatever number is agreed on and perhaps it could sneak into 4.6 as it so trivial? (For me 20 would be enough but perhaps Max Hunter would want 100).
However, I actually think removing the limit altogether may be a better option. (Perhaps an increase to the limit for 4.6 and remove the limit altogether for 4.7?)