With the Ubuntu 10.04 LTS release schedule that has just been posted to the Ubuntu Wiki there are just three alpha releases, but two beta releases. There will also be one release candidate before the final release is made in April of 2010.
The first alpha release for Ubuntu 10.04 LTS is planned for the 3rd of December followed by the second one coming after the Debian import freeze in early January. The third and final alpha release for Ubuntu Lucid is set to arrive on the fourth of February, just days after the release of Ubuntu 8.04.4 LTS. The feature and user-interface freezes for Ubuntu 10.04 is set for late February.
Moving to the beta series, the Lucid Lynx will have its first beta release on the 4th of March followed by the second beta on the first of April (yes, really). Finally, the Ubuntu 10.04 LTS "Lucid Lynx" release candidate should be out on the 15th of April and then the final release will hopefully arrive on the 29th of April.