Counting number of occurrences in column?

Try:

=ArrayFormula(QUERY(A:A&{"",""};"select Col1, count(Col2) where Col1 != '' group by Col1 label count(Col2) 'Count'";1))

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22/07/2014 Some time in the last month, Sheets has started supporting more flexible concatenation of arrays, using an embedded array. So the solution may be shortened slightly to:

=QUERY({A:A,A:A},"select Col1, count(Col2) where Col1 != '' group by Col1 label count(Col2) 'Count'",1)

A simpler approach to this

At the beginning of column B, type

=UNIQUE(A:A)

Then in column C, use

=COUNTIF(A:A, B1)

and copy them in all row column C.

Edit: If that doesn't work for you, try using semicolon instead of comma:

=COUNTIF(A:A; B1)