Lake Windermere is a modern usage that particularly annoys pedants and oldtime Cumbrians: it ws invariably just Windermere till the tourist inustry stated kicking in.
    As ot why only one lake. Well, why not, I guess. The lake district has had a lot of linguistic input, from assorted Celrtic, English and Scandinavian invaders. The Scandinavians( chiefly Norse judging by the place name evidence) seem to have predominated, and their words wer e "water" for a big lake, "tarn" for a smallish one, and "dub" for a teensy little puddle."Mere" seems to have been the main English word for a sizable bit of water at the time placenames were being allocated. coinage, Whatever the lake word was in the original Cumbrian(Welsh), it doesnt seem to have survived, unless it changed to the more Angliciesd Lake of Bassentwaite: what is the Welsh for lake? Presumably something similar, it's Lough in Irish and Loch in Scottish Gaelic. It must be something simliar in Welsh.