If theres 7 days how many nights is that?

Very impotant for a trip i need to book a hotel.

That would be 6 nights.
You do not count the day that you arrive.

5 nights.... probably. it depends on when you arrive and when you depart.

if you base it on weeks, in 7 days their area 6 nights simply because, on the following day of the 6th night you will return to the first day of the week again. get it?

Six nights. Let say you arrive in Vegas on Monday at 10 am and leave Las Vegas Sunday at 10 am, you would have been in Vegas 7 days, even if they were not full days. The nights refer to the nights you paid to have a hotel bed to sleep in. Since you are checking out on Sunday morning you did not pay to sleep there Sunday night, therefore 6 nights.

If you arrived Monday night it would still count as a day if you paid to have a hotel room that day. Any time of the day that you have access to your hotel room is a day. Any night that you can sleep over is a night. So if you arrived Monday night at 8 pm and paid to have access to a room on Monday and then checked out Tuesday morning, that would count as two days (Monday, Tuesday) one night (Monday).

It depends on how you setup your trip. For example, most trips are setup as 7 days and 6 nights. However, you can set it up to be 7 days and 7 nights, leaving on the 8th day, which for some reason isn't counted.

6, normally.

Presumably you arrive on day 1:

Day 1 (arrive)
Night 1
Day 2
Night 2
Day 3
Night 3
Day 4
Night 4
Day 5
Night 5
Day 6
Night 6
Day 7 (depart)

Note that Day 1 and Day 7 usually aren't full days, since some of them are spent travelling.

SIX nights, you arrive day one night one, day two night two, day three night three, day four night four, day five night five, day six night six, day seven YOU leave.

You usually cannot check into a hotel until after 3:00 p.m. and have to check out by 11:00 a.m.

Seven days is normally six nights.