Comprehension:
Ananya Raga, Bhaskar Tala, Charu Veena, and Devendra Sur are four musicians. Each of them started and completed their training as students under each of three Gurus - Pandit Meghnath, Ustad Samiran, and Acharya Raghunath between $2013$ and $2024$, including both the years. Each Guru trains any student for consecutive years only, for a span of $2,3$, or $4$ years, with each Guru having a different span. During some of these years, a student may not have trained under these Gurus; however, they never trained under multiple Gurus in the same year. In none of these years, any of these Gurus trained more than two of these students at the same time. When two students train under the same Guru at the same time, they are referred to as Gurubhai, irrespective of their gender.
The following additional facts are known.
- Ustad Samiran never trained more than one of these students in the same year.
- Acharya Raghunath did not train any of these students during $2015-2018$, as well as during $2021-24$.
- Ananya and Devendra were never Gurubhai; neither were Bhaskar and Charu. All other pairs of musicians were Gurubhai for exactly $2$ years.
- In $2013$, Ananya and Bhaskar started their trainings under Pandit Meghnath and under Ustad Samiran, respectively.
In how many of the years between $2013-24$, were only two of these four musicians training under these three Gurus?