I don't know if there is a limit, beyond your system resources. Longer animations naturally require more memory.

The counter only displays three digits (up to 999), but you can have more than 1000 frames. Click on the counter to enter a 4-digit number manually, if necesssary.

When you import a sound file into Poser, the graph also displays the sound's waveform in blue, as a visual aid for manually syncing an animation with the sound. Stereo sound files have two waveforms, of course. This is probably what you're seeing. If it bothers you, you can delete the sound ...

Animation menu --> Clear Sound

... while you're editing, and then re-import the sound from the File menu when you're ready to render your animation.

Poser scales the graph in relation to the length of the animation. This is not necessarily a good thing, especially with long animations, when you're trying to edit keyframes. Grab the ends of the sliders and drag to increase/decrease the graph scale.