Dear student,

Josaa will display the vacant seats left before every next round counselling on it's official site. So you can get idea of vacant seats before starting of next round counselling.

Also there is a seat matrix issued by josaa so that students get to know that how many seats are available in the institutes.
So, you can calculate it manually too, by subtracting the seats alloted in previous round from the total seats available.
So, don't worry, josaa will release list for number of vacant seats before commencement of each round counselling or seat allocation.

Check out the following link to know different dates of josaa counseling :-
https://engineering.careers360.com/articles/josaa-counselling