The D300’s built-in interval timer appears to be limited by its interval/shot-count settings, so with the camera alone you may hit a 999-frame cap. In practice, the total is determined by the number of intervals multiplied by shots per interval, and the interval count is capped.

If you want effectively unlimited shooting, the usual solution is an external intervalometer/remote timer. Some third-party timer remotes let you set the shot count to unlimited (often shown as “--”), so the camera keeps taking frames until you stop it, the battery dies, or the card fills.

So: for more than 999 frames, use an external intervalometer rather than relying on the D300’s internal timer.