IMHO, the answer depends more on the lifestyle than on the number of speakers. As long as everyone lives in a homogenous community along with their grandparents and parents, the new generations learn the language. When people move to larger towns where the majority speaks a dominant language, their children at best speak the minority language at home while the grandchildren rarely acquire it at all, and its death is imminent. Anecdotal evidence follows.
* Manx. In 1874: 16,200 speakers (30% of the population of the Isle of Man); in 1974: last native speaker dies. [0]
* Belarusian. In 1999: 36.7% of Belarusian citizens spoke it at home; in 2009: 11.9%. [1] Remarkably, it is better preserved among the Belarusian minority in Poland which is concentrated in a few rural communes.
* Lower Sorbian in Germany. In the 1880s: 70 thousand; in the 1990s: 7 thousand; around 2010: 2 thousand. Folk dances, costumes, and customs will outlast it like an empty shell outlasts its resident.
[0] https://en.wikipedia.org/wiki/Manx_language#Number_of_speake...