We had a similar discussion here in 2015 see http://www.chesspub.com/cgi-bin/chess/YaBB.pl?num=1443629168/7%5Cx237.

I really like the idea of having one general "repertoire", with various tweaks in the subvariations to keep things fresh, without entirely changing the nature of the position.

For the last few years, my White repertoire has been centered around the d3 Italian, the Bb5 Sicilian and the French Tarrasch.

In the Italian with 3...Bc5, I started out playing the plan with delayed castling and a quick Nd2-f1-g3. Gradually I added to my arsenal, and now choose between the former, the classical main line, and the modern plan with a2-a4.

In the Sicilian with 2...e6, my main line was 3 d3 Nc6 4 g3, but I've also essayed the nuances 4 c3 and 3 g3.

Within the French Tarrasch (after, in my games, the near-ubiquitous 3...Nf6) I've played the Main Line with 7 Ne2, the 7 Ngf3 gambit, as well as Yvseev's 4 Bd3.

It's nice; you can keep playing "your" openings that you understand, but still avoid complete predictability.

Ideal for amateurs is probably a deep enough repertoire that has the scope to be broad without too much trouble.  I mean, one can play the Sicilian Defense their whole career, and change variations within it (from the Dragon to the Scheveningen to the Kan to the Taimanov...), which probably takes less work than learning another whole opening like the French.  Even if you switch from the Dragon to the Scheveningen, many of the key strategic themes are similar, and you can use most (if not all) of your anti-Sicilian repertoire. 

The same is true with most other big, main defenses.  If you specialize in the Queen's Gambit Declined as Black, for instance, you can change between the Tartakover, the Lasker, and the Orthodox; then maybe add some lines of the Semi-Slav depending on move order, then maybe expand things to include the Ragozin at some point, perhaps the Vienna variation, perhaps the Tarrasch at times... you can have a fairly large, varied repertoire and be fairly difficult to prepare against without jumping all the way to the King's Indian, Grunfeld, Dutch, etc.

Of course you might want to vary things up completely occasionally for variety or fun or your chess education, but that sort of approach seems the "best" for long-term results.  You do have to sort of fall in love with a particular opening (like the Sicilian), though.  And some openings are easier to fall in love with than others!

Broad&Deep is the ideal answer. 

On amateur level both approaches should work, while it's increasingly rare to see titled players relying on only a limited repertoire that they know in depth.

I remember Malakhov once saying that his approach as Black was to stick to one defence at a time (against each first move, obviously) and just know it better than everyone, while as White it was more important to be flexible and unpredictable in order try to catch your opponents where you think they are weakest. Presumably the logic is that White doesn't get a meaningful advantage anyway, especially if Black gets a kind of position he wants and knows, so a psychological "play the man" approach is needed, while as Black it's more important to be theoretically OK and really familiar with the positions in order to reduce the risk of losing.

I do this to some extent in that I vary quite a bit with White (with 3-4 different first moves, though 1.d4 more often than the others), pretending that I actually know enough to pull that off without bluffing! Of course the point is the same as Malakhov's: To look at the database, find some weakness in my opponent's repertoire and/or style, and try to exploit that.

As Black I ideally want to have at least two defences against both 1.d4 and 1.e4, one sharper and one more solid (Malakhov's singular approach may work less well if your only defences are really sharp and unbalanced). But the reality is that my sharp defences have stayed with me over the years, while I keep cycling through more solid/mainstream defences without settling on any long-term. This lack of solid defences to fall back on, ones I really know and understand, hurts me against stronger players...

Apart from this talk of the ideal approach, in reality I find it hard to resist looking at whatever openings I find interesting at any given time. Recently I've had a French Defence binge despite officially having given it up (except in blitz) and having two other defences to 1.e4 I do well with. This forum also triggers my undisciplined opening studies. Well, sometimes I just have to follow my interest instead of fighting against it.