Anne's GTS site pages on the Sowerby Butterfly 2552 set at http://www.glasstrinketsets.com/cms15/index.php?option=com_content&view=article&id=53&Itemid=107
only shows one lidded pot in the Sowerby catalogue , the set consisting of a tray, pair of candlesticks, a single lidded pot, and pintray - 5 pieces (the lidded pot counting as one piece).
Certainly I would only count a lidded pot as one piece rather than two.
I can't recall having seen 2 sizes of lidded pot with a Sowerby Butterfly set, and a quick scan of current and completed ebay listings only show sets with one lidded pot.
As to the 'usual' number of pots in a set - there is no 'rule' as far as I am aware. The Sowerby 2487, 2501 and 2638 sets are only shown in the catalogue as having a single lidded pot. Other sets have 3 pots (usually, but not invariably, 1 large and 2 small). Many other sets have 2 lidded pots (in 2 different sizes).
The pin dish/tray is often seen on eBay either on its own or with other pieces of a part set. The Sowerby butterfly set is common in all the colour combinations so the completion of a mint set using odd pieces should not be a difficult task. good luck.