According to official sources, there are 1270 natural and artificial water basins in Slovenia. But, in private conversation, people who deal with studying and counting of Slovenian water supplies, give the number up to 10000 of lakes and water basins of all kinds.
For a small country of 20000 km2 it's a huge number, even if we mind that only less than 300 of those 10000 can be actually (scientifically) named “lakes”.
The largest of all is Cerknica lake, which surface may rise up to 30 km2. But, there is a peculiarity: the lake is of karst origin and completely disappears for months. The water comes up, from underground hollows and fille in a basin for 7-10 month per year, but may completely disappear during the dry season.
The largest permanent lake is Bohinj (approx 4 km length and 1,2 km width), which is a glacier lake on the territory of Triglav national park. Bohinj is quite popular destination with lots of possible activities all year round.
There are quite unusual lakes, like Divje (Wild) lake. Quite a small basin (40x50 meters), at first sight, it appears to be the deepest lake of Slovenia with 160 meters of explored debth and a huge flooded karst well.
And the most visited is, of course, Bled lake one of Slovenian iconic symbols. Incidentally, the only one Slovenian island is located here, on Bled. It's so called Bled island with Assumption of St. Mary Church on it. This place is popular all year round, that is why at low season and during the workdays walking the banks is simply charming.