For land snails, see Solem et al (1981) Sympatric species diversity of New Zealand land snails, New Zealand Journal of Zoology 8:453-485, and articles citing that article.

http://www.tandfonline.com/doi/pdf/10.1080/03014223.1981.10427971

I have found some of this but of course it depends on where is the ecosystem because tropical zones will clearly have a higher numbers than non tropical...as an example I was accustomed to have more than 2000 species in Cuba to work with...and now here in Spain I rarely see land snails, and there are just a few species...

https://books.google.es/books?id=_d8nmKrIxXIC&pg=PA60&lpg=PA60&dq=land+snails+biodiversity+numbers+cuba&source=bl&ots=1gmsADv-q3&sig=BEfNziO-1Ol0xDhke2_KpYIjHv4&hl=es&sa=X&ei=YKHUVKGHBInnUp_TgvgF&ved=0CFsQ6AEwBw#v=onepage&q=land%20snails%20biodiversity%20numbers%20cuba&f=false

This is a question I have worked on for a long time! I guess you will have fished out papers from Researchgate, but I attach a summary one just in case not. The crucial issue here is the scale at which you ask the question. Referring to Jonathan's answer, there are actually more than 300 species in Spain (Ok, not up to Cuban standards, but a lot more than in Britain or Belarus), But in N. Europe we have local faunas of up to 50 species in areas of less than 1 ha. Not as rich as the richest tropical sites but not far off, and in most tropical areas or even in the Mediterranean, local faunas are poorer than those in damp deciduous forests of the north, even though the faunas are all post-glacial immigrants. With Beata Pokryszko, I found rich faunas in the Polish part of Bialowieza forest, and I suspect you would find the same in the Belarus part. Tropical or mediterranean diversity is very high because there are many very restricted endemics, not because many more species live syntopically.

Robert Cameron