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You can read in the first 500 elements like this:

data = ToExpression@ReadList["myfile.txt", Record, 500, RecordSeparators -> " "];

On Linux/OS X, items 500 to 1000 can be read in this way:

n = 500; m = 1000;

data =ReadList["!cat myfile.txt | tr ' ' '\n'  | head -" <> 
          ToString@m <>" | tail -" <> ToString[m - (n - 1) ]];

Assuming your record separator is a space, and that you might want to ignore records which are not of the form, {...}, you could use the following to find records n through m:

n = 1; m = 500;

data = ReadList["!cat myfile.txt | tr ' ' '\n' | grep '{.*}' | head -" <> 
  ToString@m <> " | tail -" <> ToString[m - (n - 1) ]];