According to the answer of this question the Force sensitivity is genetically inherited. I.e. you are more likely to be eligible for Jedi training, if your parents were.
The Jedi code forbids having children and the Jedi Order existed for several thousands of years. Did this lead to reduction of the potential Jedi / Force sensitive individuals?
No, it did not, because the Jedi weren't the only Force Users in the universe.
The Jedi were the best known, constantly sticking their nose into galactic politics as Jedi Knights, but there was also the Sith, the Force Wielders, the Force Priestesses, the Nightsisters of Dathomir, and the Dagoyan Order.
While the "main Sith" had their rule-of-two business going on, even the "main Sith" weren't the only Sith (at least in Legends, the "Rule of Two" Sith were looked down upon by other orders of the Sith; they're mentioned in the New Jedi Order books).
Force-Sensitivity was also a variable thing; Jedi had amazingly strong powers, but anyone could be a "little bit sensitive." This is expanded on in the EU; characters would have "eerily accurate premonitions" or "surprisingly fast reflexes". They wouldn't attribute it to "The Force," but more likely to skill or luck. But they would still carry the genes, and eventually enough "bits" of Force would stack up to cause a powerful force sensitive to appear.
Finally, depending on how you view the force, if it has a will of it's own, it would cause Force Sensitives to be born no matter what.
