n-1.
when each match is played a team gets eliminate so after 1st round n/2 teams remains
in next  n/4

and so on
till one team remains.
therefore
=n/2+n/4+…………+1
=1+2+4+………….._n/4+n/2
this is a GP sequence
a=1 r=2
and no. of terms = x(assumed)

after each match, one team is eliminated. thus after first round, n/2 teams are left.
after 2nd round , n/4 teams are left

and so on

after k th round

n/(2^k) teams are left

Now for n/(2^k)=1 team
n=2^k
k=log(n)
a=1
r=2
k=log n (base 2)

total matches = a(r^n – 1)/(r-1)
=2^(log n) -1
=n-1

hence answer is n-1