If the pulse width is 85 microseconds, there won't be much of a gap between them at 12kHz.
If you are using a Uno etc, you are limited to 1 16 bit timer and 2 8 bit timers (really only one of which can easily be used).
What level of granularity do you require ? Would the following example be fine enough :
Frequency Steps (KHz) Period (uS)
--------------------- -----------
12.5 80
10.0 100
8.33 120
7.1 140
6.25 160
etc. etc.
If so, you could probably get away with one timer. However, if you require a continuously variable frequency, it will clearly not be sufficient.