When we see a title that ends in Teaser 1 or Teaser 2, the number is a sequence marker added by whoever uploaded it. It tells you the order in which that uploader posted teasers for the same release. It does not tell you how many teasers exist in total, and it does not say that one teaser carries more weight than another. A Teaser 2 can sit beside a Teaser 1 and a later Teaser 3, or it can be the last one in the set. The number counts postings, nothing more.
A title that says Teaser with no number is a different case. Usually that points to a single teaser for the release, since there is nothing to put in order. Usually is the honest word, though, because some uploaders leave the number off the first clip and only start numbering once a second one arrives. If you want to be sure, open the uploader's recent posts and look for other teasers for the same release posted nearby. Treat the missing number as a hint about the count rather than proof of it.
Now the parentheses. A tag such as (Prologue ver.) names a version of the track or clip, and that is all it says by itself. It does not tell you how that version differs from the final release, whether it is shorter, rearranged or simply cut differently, and we should not guess. Think of it as a label for telling two uploads apart. If the description spells out the difference, take that explanation exactly as written. If it does not, the tag stays an unexplained name.
The part worth putting in your calendar is the release date line in the description. It is often written as Year.Month.Day, and sometimes it comes with no time of day. When no time is given, do not assume one; save it as an all-day entry and update it only when the uploader states a time. To confirm what you are looking at, read the description lines for artist, title and release, then check that the uploading channel is the official one for that act. Labels vary between uploaders, and a label never tells you what is inside the clip.