Leonardo-based tachometer?
First, make sure that your LCD routines and wiring are working, by trying to display something on it.
Second, the code isn't correct - it will have some fluctuations from one measurement to another.
One way to do it is to start timing only after the first pulse has arrived, and stop timing only after the last pulse has passed.
Something like this:

Wait until a falling edge.
Time0 is millis().
Wait for N pulses to pass.
Time0 is millis() minus time0.

Now with time0 and N you can calculate your rpm.
Edit:
Here is the above code at work, running Arduino on a PIC24.

The input signal is 10Hz, 1 pulse per revolution, aka 600 rpm.
As you can see the rpm read-out is precisely 600rpm. I'm running the code at 8x oversampling.
As I indicated earlier, your code will introduce uncertainty, as much as one period for the pulses. So in a single pulse measurement, the rpm can be anywhere from 0.5x of the true value to 1.0x of the true value.