Two (2)
The proof that 1+1=2 is a famous one from the book Principia Mathematica. The proof is actually completed in Volume II, 1st edition, page 86, accompanied by the comment, "The above proposition is occasionally useful." They go on to say "It is used at least three times, in *113.66 and *120.123.472.". The theorem above, ∗54 ⋅ 43 ∗ 54 ⋅ 43, is already a couple of hundred pages into the book. The main reason that it takes so long to get to 1 + 1 = 2 is that Principia Mathematica starts from almost nothing, and works its way up in very tiny, incremental steps.