ok. Question then should I not just focus on 1 character at a time?
Well, we are not penalising players if they want to try multiple routes at the same time, but when they will start dating one character, other routes won't be available
There are however optional events which are available regardless of the routelock status. MC scenes are sorts of like side stories with stories involving Kei's slice of life outside of route, including stories focused on the main protagonist, events with a main cast and exploring characters like his sister Tomo or faculty. They also adapt to players' playthrough and are aware of their routelock status, offering unique dialogue based on which route you're playing. So yeah, you can play them without being worried about anything. The scene you have listed (MC012) is also a size 3 event
I'm one of the devs, I need to know this type of things lol
Seriously though, we don't have official guides with tables listing each of the sizes implemented in game yet (as the game is still WIP), but for main routes their sizes tends to follow this pattern:
001 - 009 is Size 1; 010 - 029 is Size 2; 030 - 049 is Size 3; 050 - 069 is Size 4; 070 - 089 is Size 5; 090 - 100 is Size 6
These numbers are arbitrary though and can differ between routes with plus / minus a few scenes as while 100 scenes is a minimum target for each main route, some of them will have more than 100 scenes. Optional events will have much smaller scene count, so their numbers for each of the sizes may differ
As for MC scene you listed, the unlock condition for MC012 is finishing global032 (a mandatory event informing players about current Naomi's predicament) and this one is very early size 3 scene. MC012 uses some story beats from this scene as a background for its story