Two things to keep in mind. First, we don't know exactly how these will be acquired, beyond them not being pure RNG world drops. Second, we're starting with a cap of 1 legendary equipped at a time, with various gated ways to slowly increase the limit over time. For example, fully upgrading your Class Hall (which will take a month or two) adds +1 to the cap. Other ways will probably take longer, or not even be available until future content patches.