C (gcc) -lm, ^{(削除) 23 (削除ここまで) \$\cdots\$ (削除) 50 (削除ここまで)} 27 bytes

Saved 6 bytes thanks to a'_'!!!
Added 38 bytes to fix type error kindly pointed out by S.S. Anne.
Saved 3 bytes thanks to rtpax!!!
Saved a whopping 23 bytes thanks to an idea from ErikF!!!

#define f(n)ceil(n/sqrt(2))

Try it online!

C (gcc) -lm, ^{(削除) 23 (削除ここまで) \$\cdots\$ (削除) 50 (削除ここまで)} 27 bytes

Saved 6 bytes thanks to a'_'!!!
Added 38 bytes to fix type error kindly pointed out by S.S. Anne.
Saved 3 bytes thanks to rtpax!!!
Saved a whopping 23 bytes thanks to an idea from ErikF!!!

#define f(n)ceil(n/sqrt(2))

Try it online!

C (gcc), ^{(削除) 23 (削除ここまで) \$\cdots\$ (削除) 50 (削除ここまで)} 27 bytes

Saved 6 bytes thanks to a'_'!!!
Added 38 bytes to fix type error kindly pointed out by S.S. Anne.
Saved 3 bytes thanks to rtpax!!!
Saved a whopping 23 bytes thanks to an idea from ErikF!!!

#define f(n)ceil(n/sqrt(2))

Try it online!

C (gcc) -lm, ^{(削除) 23 (削除ここまで) \$\cdots\$ (削除) 50 (削除ここまで)} 27 bytes

Saved 6 bytes thanks to a'_'!!!
Added 38 bytes to fix type error kindly pointed out by S.S. Anne.
Saved 3 bytes thanks to rtpax!!!
Saved a whopping 23 bytes thanks to an idea from ErikF!!!

#define f(n)ceil(n/sqrt(2))

Try it online!

C (gcc), ^{(削除) 23 (削除ここまで) \$\cdots\$ (削除) 53 (削除ここまで)} 50 bytes

typedef unsigned long long L;L f(L x){x/=sqrt(2);}

Try it online!

Saved 6 bytes thanks to a'_'!!!
Added 38 bytes to fix type error kindly pointed out by S.S. Anne.
Saved 3 bytes thanks to rtpax!!!

C (gcc), ^{(削除) 23 (削除ここまで) \$\cdots\$ (削除) 50 (削除ここまで)} 27 bytes

Saved 6 bytes thanks to a'_'!!!
Added 38 bytes to fix type error kindly pointed out by S.S. Anne.
Saved 3 bytes thanks to rtpax!!!
Saved a whopping 23 bytes thanks to an idea from ErikF !!!

#define f(n)ceil(n/sqrt(2))

Try it online!