Look ahead

There is no hard number.

There could be only 2 to consider on one move and a dozen in another case.

You need to look for ALL threats.  If one of your pieces is holding the entire position together, such as the Knight on e2 in the Exchange Grunfeld, classical line with 7.Bc4 and 8.Ne2, then one thing you must ALWAYS consider, if he has a legal move to do it, is what happens if he takes that key defender.  It can easily be a sacrifice.

For example, go to http://chessstream.com/Land-Of-The-Sky/ and click on games.

Select PATRICK MCCARTNEY vs PETER JOHN LIOTINO.

It will start moves automatically, but you can stop that and move manually by clicking on the circle with a square in it under the board and then use the arrows to make moves.

Go to the position where it is Black's turn to make his 31st move.  You need to observe patterns.  Black should probably trade Knights.  Instead, he goes 31...Ng7, which is not good.  Notice that it no longer guards e7.  The squares e7 and d8 can suffer a Knight fork from c6, and the knight on g7 is loose, and in the line of fire of the White Queen once the Knight moves.  All of this leads to White's winning combination, and my 32nd move had to be considered and probably wasn't.

32.Rxe7!! Kxe7 (If 32...Qxe7, then 33.Nxc6 wins back the exchange and White is up a pawn for nothing.) 33.Nxc6 Qxc6 34.Qxg7 and I had to see this when I played the 32nd move.  Black cannot stop 35.Re1 and against all moves except the game move, the f7 pawn will fall as well, bringing material count back to even, but with Black completely busted.  After the game move, the back rank weaknesses are fatal.

34...Rd7 35.Re1+ Kd8 36.Qf8+ Kc7 37.Re8 and Black is dead.  He immediately gave up the queen without a fight, but even after something like 37...Qb7, White has 38.Ra8 with the fatal threat of 39.Ra7 (do not forget about that Bishop on f2 that also prevents desparado checks by Black.  If the Queen moves away, the king will get mated.  If it does not, it will be pinned to the king.

So as you can see, there is no simple answer to it.

From that same site, if you look at Patrick McCartney - Rochan Bakthisaran, you can again see that after Black's error of 11...Be7 (11...f6 is correct), many moves that are not the most obvious are played.

For example, 19.Qc2 instead of 19.Qa4, despite being in the line of fire of the rook on c8.  It was all about getting to the kingside quickly.  From a4, the queen does nothing beyond pinning the knight.

Move 24 White, you cannot just assume a recapture on b4.   I did recapture, but not until move 30!

Nobody ever said chess was easy!  Want proof?  Look at the lemon I played in the final round, the other game on that site where I had White.  Just because I played brilliantly rounds 2 and 4 does not guarantee success in round 5, despite having White those 3 rounds.