Here is something a little bit more systematic. The following code generates Wolfram|Alpha queries with different numbers of "a's".

results = 
  Monitor[Table[{i + 1, First@(List @@ WolframAlpha[Evaluate@StringJoin[Join[{"solve b=a"}, Table["+a", {i}], {" for a"}]], {{"Result", 1}, 
 "FormulaData"}])}, {i, 1, 40}], i];

It turns out that the interesting one is at the transition at 24 a's. The following runs 40 trials of that one request.

results2 = 
  Monitor[Table[{i,First@(List @@ WolframAlpha[Evaluate@StringJoin[Join[{"solve b=a"}, Table["+a", {23}], {" for a"}]], {{"Result", 1}, 
  "FormulaData"}])}, {i, 1, 40}], i];

Here are the results in two tables:

TableForm[{{Grid[Join[{{"Number a's", "Result"}}, results], Frame -> All], Grid[Join[{{"Trial", "Result"}}, results2], Frame -> All]}}]

Cheers,

Marco

what about 25 or 30? i should have mentioned earlier, i did get inconstant results, sometimes it would accept 24, but not 25, sometimes it would accept 23 but not 24 etc