This all comes down to
How do you store your levels?
How do you need your objects to behave?
What is the game?
Do you care if an enemy resets once it goes off screen?
Do your levels mostly follow 1 direction but you swing out to the other direction, I.e Mario 3 is mostly L and R but you can go up and down, Radian is mostly U and D but you can scroll L and R a bit?
In those cases you can store all your entities in an array from L to R. Then you keep a Head and Tail index. And each ent def holds a number of chars till next ent.
so
Type 00,14,02,05
Dist 00,05,04,01
So this way you add active entities from the Head, and when Tail moves over one, you remove the entity from being active( unless your entities can move and follow you across screens, like a boo for example at which point you only cull them once they are "out of screen" where out of screen might be screen size + extra)
If you count Chars or Blocks or Screens is up to the game design.
Which is basically the Anchor system tokumaru mentions but you have a Left Anchor and Right Anchor portion to the "window" rather than just a start.
Once you have entities you keep them in Screen Space, and cull as needed. If you are moving Left then you spawn the new enemy off screen on the L, if you are moving R you spawn them off screen on the R, so you don't need to work out where they are, no maths
For speed you can keep a OnScreen List and then a OffScreenButActiveList, the first gets full update and animation, the 2nd gets position only and no animation logic, you can also if your system handles it lower their update and collision rate to keep the frame rate rolling.
For something like Gauntlet that won't work, and you start to get into a Quad Tree like activation zone tables, or you carve up things in to large blocks and then add to a active block list, then you keep indexes into the position of those blocks, updating the list as you cross block boundaries.
Or you go with "volumes"/"zones" where you place down some special chars, or an AABB list that you can quickly detect against. Then you put triggers on the zones, so when the player enters the Zone, you spawn A,B,C and X,Y,Z etc then as the player moves around they enter the volumes/zones and that controls what objects are made. Used in things like RPGs where you have doors and towns and save points etc