There is no complete list of Nintendo 64 roms on the internet, there is a database called Goodn64 which categorises some of the roms you can find on the internet as well as other places... I do not remeber the exact number but it contains around 1800 entries. If you multiply every known rom with one million, maybe more, maybe less, you will get an estimate. But there is really no way to know how many Nintendo 64 roms there are floating around the internet.