Major kit vendors, including Winexpert, state that topping up is not necessary IF the kit is bottled on their schedule. This assumes carefully racking so a 23 liter carboy in down 1 to 2 bottles worth (750 ml to 1.5 l). Folks on this forum have validated this, so I believe it to be true.
However, the general recommendation on this forum is to NOT bottle on the vendor's schedule, to bulk age the wine at least a few months. In this case, you want to topup within a few inches of the stopper. The first choice is a compatible wine, either the same variety or one that blends well. I recommend using a wine of quality equal to or greater than what's in the carboy -- when blending you are limited by the quality of the lesser wine.
Second choice is any wine.
The goal is to protect your investment. By this, I don't mean just monetary value. Investment includes your time, effort, pride -- whatever the wine means to you.
Other options? One is to use multiple containers -- I have a bottles from 125 ml on up, with drilled stoppers that fit them, plus as drawer full of airlocks. Another is to use marbles, although you need to ensure they are lead-free. Some folks use insert gases, although I don't trust that as there's no way to tell how much of that space is air and how much is the inert gas.
Purity of wine used to matter to me. I consider that a large chunk of the world's wine is blends -- outside of Burgundy, most French wines are blends. Same is true of Italy and Spain. In the USA, a wine may be labeled with whatever grape comprises at least 76% of the wine, e.g., I can make a blend of 76% Cabernet Sauvignon and 24% Thompson Seedless, and call it Cabernet.
I used multiple containers for many years, and in some cases still do. However, for kits, I top 'em back up to 23 liters with a compatible wine as it's a LOT less hassle.