There's something wrong with your function in that case. A number field can store values greater than 32767 (the limit for integers in Notes), so your code needs redress.
I created some test code to increment a double which works just fine. If you're getting a "Type Mismatch" you're probably not setting the "starting point" for your code properly (i.e. the value you're grabbing from the document isn't registering in script as a Double):
Dim dbl As Double
Dim dblNew As Double
' // Instantiate doc NotesDocument
dbl = doc.YOUR_NUMBER_FIELD(0)
dblNew = dbl + 1
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