What happens if you exceed the GBs limit of 10 sprites per line ?

When you exceed 10 sprites per horizontal pixel line, only the sprites with the highest priorities will completely appear in that pixel line. (Sprite #0 has highest priority, sprite #1 next highest, etc.) The other sprites will partially or totally disappear (depending on how many of lines of each sprite break the 10 sprites-per-line limit.)

If you plan to have more than 10 sprites that at some time will share the same lines, then continuously alternate some of the sprite priorities with each vblank. This will cause them to blink annoyingly (but only when the 10 sprites-per-line limit is exceeded) but this is often better then them disappearing partially or completely.