You can easily get strings of line-powered LEDs up to 400 or more, or shorter strings which can be connected end-end i.e. Christmas lights. They effectively are series strings connected in parallel. These are usually very power-efficient but usually shows some flicker; some people are more sensitive than others. And of course you need some place to plug it in, and need to take usual line voltage precautions.
I've seen battery-powered strings up to 200 or more but at some point the drain is too much to make it practical even with D cells. These are parallel wired, likely lower efficiency than series. Whether you can get 300 in the LED colour/style you want, would have to check around, and this long after Christmas is not the ideal time for stores (so online better).
You could hand-wire 300 LEDs, but pre-wired strings would be a whole lot easier. Challenge with low-voltage parallel is getting uniform current-sharing and brightness along the length; using one dropping resistor per LED would be a fun job.
Dave