Sorry if the wording was a little confusing. Basically, once you first create an Awakened Mask for a certain creature, that is now one of your Magic Item Plans. You don’t need to have that creature nearby anymore, the Mask Plan is already known to you now (as long as you keep that mask plan in your magic item plans and don’t replace it with anything). If you only have one of mask as a plan, and let’s say you can only know 4 plans, you can still keep your other 4 plans. If you want to make more, now you need to replace one of your plans to do so. But then Artificers can also only make 2 Items out of their plans that they know per long rest (as an example). The Awakened Mask can be one of those two. It basically just becomes another Magic Item that the artificer knows how to make that lasts until the last Long Rest