if its occurs 1 then only one record will be copied to the 
internal table among the total number of entries selected 
as the selection is restricted by the internation table.

if its occurs 2 then only 2 lines would be displayed or 
fetched to the internal table.

if you know the number of lines then you can specify that 
in OCCURS statemnt that would take that much of space but 
if not sure then its better to take OCCURS 0.

Let me correct u Siddharth, 
   Its not correct that when u use occurs 1 or occurs 2 u 
cannot append more values than that, if u use that memory 
up, system automatically allocates enough memory so that 
the internal table can hold more records.
       And as far as occurs 0 is concerned when u use 
occurs 0, system allocates 8kb memory from the paging area 
unnecessarily, if u intend to retrieve less rows then its 
better to write some value on occurs clause.
   There is only one case where occurs clause limits the 
number of rows that can be added to an internal table, i.e, 
when we use APPEND SORTED BY.