The number of engines is primarily driven by the need for thrust, then by reliability. Larger, heavier airplanes or airplanes that need to carry more weight (F-14, F-15) need two engines, where smaller airplanes like the F-16 need only a single engine. Single engine airplanes are smaller, lighter, simpler, cheaper to manufacture, operate and maintain.
While two engines do provide increased reliability, they also have twice the chance that something will fail. Jet engines are reliable enough where having the second engine is not a major player in airplane design. Arguably, naval aircraft have the highest need for reliability.
Advantage of single engine over twin is simplicity and lower cost. Also, a bit of efficiency, as one large engine is usually more efficient that two engines of exactly half the power, all other things being equal.
This has nothing to do with speed. For supersonic flight, the most important aspect is aerodynamics related, essentially in the presence of a suitable engine air inlet (most of the thrust in a supersonic aircraft comes from the inlet, the engines is essentially a pump to relieve the internal pressure. This may sound weird, but that is the way it is).
If one wants to install double the power in an otherwise identical aircraft, the plane may be able to get to speed faster, but the top speed may not change that much, as it is usually dominated by other restrictions, materials for instance. The F-104, for instance, among a few other single engine supersonic aircraft, had to be restrained and throttled down at high speed, as it could have otherwise reached speed that the structure could not stand, mechanically and thermally.
Having two engines is somewhat like having two hoses pulling a carriage; the horses may go faster if the load is heavy, but with a light load, they would run at their top speed, and no more.
Flying at Mach 3 means first and foremost having an air intake able to supply an engine with the right air flow at that speed. Which in turns means that the air intake will not be that great at speed less than Mach 3. Unless it is a variable intake, which is a complex, heavy and costly solution.
And then you have to have an airframe (wing and fuselage) that could fly well at Mach 3, but also is able to fly reasonably well at 150 knots, because the plane still has to be able to take off and land at a reasonable speed.
Super high speed is not of a very good use for fighter. At Mach 3, evading a missile (turning) takes a couple of Zip code area. Moreover, since the airframe is basking in its own kinetic heating, it becomes an interesting infra-red emitter, which some missiles would have little trouble locking into (i.e. it is not only the engine that are emitting heat).
Mach 3 makes sense if you are sneaking above (spying), or dashing through (bomber) at an altitude where interception is unlikely. For dogfighting, what is needed is large wings and high thrust to weight ratio, to provide with quick acceleration and fast turn rate.
So from above long essay, we can conclude that for dogfite or CAS or raid on Indian Targets PAF's single Engined fighter jets are capable enough to make a strong statement. But when it comes to protecting its Maritime intersts and ports, Pakistan need to have a dedicated Air wing in its Naval fleet comprised of J11B.. Why there is need some members may ask... Its due to the induction of Aircrat carriers and 48 Mig 29KUB along with Naval Tejas by Indian navy. Ignoring sea supply lines may cost Pakistan dear at the time of war.. Meanwhile I heard long back that PAF was interested in J11B as well. Whats the current status??
