A DigitalPoint Forums thread records that someone was transferred from the original Yahoo! Search Marketing product (i.e. Overture) with 22 campaigns to the new Yahoo! Search Marketing 2.0 (i.e. Panama release). When he/she tried adding an additional campaign to the pie, the program did not allow it. Reportedly, Yahoo! restricts the account to a maximum of 20 campaigns.
This is not unusual, Google AdWords restricts the campaign size to 25.
The reasoning is the same, I assume, with Yahoo! as it is with Google.
That 25 or 20 campaigns should be more than enough. That you should be easily about to use ad groups and keywords/ads within the ad groups to achieve what you are looking for out of your campaign. The past YSM was more flat, the new YSM is more relational, so advertisers will have to learn to adapt.
Forum discussion at DigitalPoint Forums.