Assuming that all of the characters are unique, interesting, and stand out from the rest of the cast, is there a limit to how many characters I can introduce in the first chapter before my reader starts struggling to remember who's who?
If not a limit, is there a certain number (or maybe a percentage relative to the total amount of characters in my cast) that is considered "good" or "reasonable" to introduce in the first chapter?
At risk of sounding glib, I would say "as many as will fit". But I think that probably is the answer. A chapter should have a shape to it. It should accomplish something. It should have focus. As many characters as fit within that shape and contribute to that goal should be fine. Sometimes that will be one. Sometimes it will be dozens.
An opening chapter has to achieve something in story terms. It has to establish a conflict or a relationship or a challenge, or something of value to be gained or lost. The characters that are essential to achieving the chapter's story goal should not overwhelm the reader.
Introducing people who don't fit just because you want to establish them for later may overwhelm the reader, not because readers have a fixed maximum for character introductions, but because outside the story arc of the chapter, they don't know what to do with them. They are information that the reader does not know how to process. That is what will overwhelm.
In general humans are able to track no more than 7 distinct factors. If you want the reader to root for everyone you introduce I think that number would be the practical limit.
From another angle, you want to give each character enough attention without the story slowing down to snail's pace. Think Tolkien: Many people do not dig him because of the very very broad scope the story follows.
Anyway if the number becomes big enough, that is exactly what you have: A group, NOT a big number of characters. If not in your mind than still in the mind of your reader.
As a general rule of thumb I only introduce the main MAIN character in the first chapter. That's to say, just one person. You can introduce someone else with them, but no more then one other person. After that, you can gradually introduce the characters at whatever interval you deem necessary. I like trios or introducing single characters by themselves, but in a scene with the main so they aren't completely alone.
In the first chapter, you need to introduce the protagonist and antagonist.
If you consider the story a "duel" between these two, you might introduce their "seconds." That brings the total to four.
There might be a fifth, (and possibly sixth) character that gives the early part of the story its "flavor." But that's pretty close to the max.
Here, I'm not counting incidental or throwaway characters. For instance, my screenplay opens with five partners of the firm, but only two of them, the heroine and the "big boss" (a second) really matter; the other three are "throwaways." The hero and his "second" are also introduced.
You involve up to five to eight characters in the beginning and add more in later chapters to which it's most preferable when their roles are smaller. Depending on the size of the story, remind the reader who they are and what they look like in casual description of what they're doing in scenes.
