The problem with Nepster's suggestion here is it doesn't in any way (whether in full, or deducible if you know the total number of lemmings) show how many lemmings are not yet spawned. This can be important for levels like, eg. Cascade. Proxima's suggestion, on the other hand, does make this information available.
So... let me see if I'm understanding this right... we'd want to have:
1. Lemmings not yet spawned
2. Save requirement
3. Number already saved
4. Total number of living lemmings
So... I'm thinking (and let me know if I'm overlooking something here)... could #2 and #3 not simply be combined into "Number needed to be saved"? This would essentially be what the "Relative In Count" option already does - it displays the number of saved lemmings relative to the goal (so for example, if you're 10 lemmings away from the requirement it'll display -10; if you're 5 lemmings above the requirement it'll display 5).
This would leave three pieces of information - Number of not-yet-spawned lemmings, number of lemmings needed to be saved (or how many more you've saved than required), number of alive or potentially-alive (in the case of not-yet-spawned lemmings, or not-yet-used cloners) lemmings.
I would think that if the captions were replaced with icons, or even simply shortened to a single letter, this should fit without having to change anything about the info panel except for what text is displayed there.
EDIT: If we assume that we do not touch anything further left than where the "O" in "Out" is in the current display, we have 26 characters to work with. Take away 6 to use for time ("T" or an icon, plus 2 digits each for minutes and seconds, and the colon seperating them), so we have 20 characters.
If we assume each value will never exceed 999 (and lag from having too many lemmings becomes an issue before this point, unless you have a very powerful PC; besides I doubt there would ever be a need for this), then we have more than enough room. In fact, even if we were to allow up to 9999, we'd have enough room. This should be doable, unless I've miscalculated.