Logic Pro automatically addresses all available system memory. The amount of RAM available for use by Sampler is determined by several factors, including:
The amount of physical RAM installed in your computer.
How much RAM other open applications and the operating system are using.
How much RAM Logic Pro is using. This varies in accordance with the number and size of audio files in the project, and other plug-ins used. Sample playback instrument plug-ins not made by Apple can significantly affect the amount of RAM that Logic Pro uses.