How many poles of electricity are in a distance of 1 kilometer?

That varies with the voltage of the lines. Higher voltage lines are kept further from the ground, necessitating taller structures which in turn can span longer distances.

Each larger structure costs more, but you need fewer of them and the minimum installation cost has been optimized for each common voltage. Here are some examples of transmission voltages:

On our system, transmission is done at 115,000 volts, and the structures are 900–1000 feet apart so there would be 3.3 to 3.7 per kilometer. On the distribution side, it depends on when they were installed.

The older ones are 12,500 or 14,400 volts and are 300 feet apart so there are 11 per kilometer, while in newer areas that we built out (or converted to) 24,900 volts, the power poles are taller and spaced 400 feet apart giving 8 per kilometer.

As a rough estimate, new transmission lines cost $1M / mile to install (US$600,000/km) and distribution lines are $300,000/mile (US$180,000/km), but rocky ground, mountainous terrain, or any water crossings can cost much more.