This is another good example of why you should keep an eye on your fluid levels. Old car or not, it's a good idea to run an eyeball over things at least once a month at a minimum. You can often catch a problem before it becomes a catastrophe or breakdown.

If the transmission is working normally after topping up the fluid, there's likely minimal damage done. Drip the fluid on a paper towel and look at it and smell of it. If it's still red and doesn't smell burnt, you're in good shape. If it's brown or smells burned, there may be some damage done to the clutch linings from slipping, but likely minimal if the transmission is working normally with adequate fluid. Change the fluid and filter and move on with life. They're pretty tough old transmissions all things considered.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]