I want to start studying a few openings. Say I am currently most interested in the King's Gambit as white, and want to play it as often as possible. (This isn't particular to the King's Gambit, I just had to name something.) Since I want to practice that opening, I obviously start my white games going
1. e4. But then black goes
1... c5 or
1... d5 or
1... Nf6 or...
Clearly I can't start studying all
1. e4 openings simultaneously, so I have to accept that some times I will be out of prep after a single move, and that game will be "wasted" in terms of practicing the opening I'm actively studying.
But I could at least study King's Gambit and, say, Sicilian at the same time, and thus be at least somewhat better covered. And then, because I will be white only half the time, maybe I should also keep one or maybe two openings in active study as black as most places where you play chess, you play black as often as you play white?
Obviously this depends on my personal capacity, and you can never know me well enough through this site to give me good, bespoke personal advice. But reflections on how this is commonly approached would be nice.