Limited quantities is a marketing term that LEGO uses in their LEGO Direct Shop at home catalogs. As a marketing term, it doesn't have a specific meaning in terms of quantity available for sale. However, the meaning that has been observed, at least in the US and UK, is that this is an item that LEGO has recently discontinued production on and wishes to clean out of inventory. That is, once the supply is exhausted it will not be replenished. It is apparently more restrictive than Hard to find or Not available in any store.
Here are some examples of items observed to carry this tag in the past:
- Bulk pack items just before they went out of stock completely
- Older sets after the theme they were in had ceased production (for example, Divers sets were tagged this way a year after they were last seen in general retail, then were gone in a subsequent catalog)
"Limited quantities" does definitely seem to correlate with whether sets are currently in production or not, it applies to things that are out of production. Some sets have subsequently been reissued, of course, but there is no guarantee of that, so if you see an item tagged this way, and you want it, move fast.