I am using multiple ultrasonic sensors(hc-sr04) with arduino for a parking project. I want to know that what is the max range arduino signals can travel because the distance between arduino and ultrasonic sensors in my project would not be small and i am afraid as the distance increases the communication between arduino and my sensor would be lost..
any guidance plx 
To play it safe, couple of centimeters. Probably overdone but you can't do meters without running into trouble.
You can consider RF communication or something like RS232 or RS422 drivers:
Use tiny Arduinos like the nano or pro micro, put one with each sensor.
@aarg Putting an arduino with each sensor would be costly..
Use ATtiny boards; you might have to add a few components, not sure.
Did not want to much time on a search, so on one of my local distributors
ATtiny44A; roughly 4 US$
ATtiny85; roughly 6.50 US$
Price only given to convert from local currency to something more international 
Shop around
A compatible Pro-Mini-Module-Atmega328-5V-16M 1.13€ free shipping
If i use a small arduino maybe (promini) with each of my sensor then how will i communicate with all the arduino network.. For example 50 sensor slots mean 50 arduinos and the distance would be atleast 5-15 meters between every arduino..
I want to know one thing that what is the maximum range two arduinos can communicate serially?
Referring to Reply #6
You need need to provide a more comprehensive description of the whole project. At the moment you are just asking questions about little bits of it with no context in which to understand the questions.
What, exactly, do you mean by "communicate serially" - how, for example, would they be connected?
How often does the communication need to take place?
How much data needs to be sent?
Is bi-directional communication necessary?
etc etc etc
...R
Danish_Javed:
@anon57585045 Putting an arduino with each sensor would be costly..
Danish_Javed:
If i use a small arduino maybe (promini) with each of my sensor then how will i communicate with all the arduino network.. For example 50 sensor slots mean 50 arduinos and the distance would be atleast 5-15 meters between every arduino..I want to know one thing that what is the maximum range two arduinos can communicate serially?
Another thing. If you are doing 50 slots, the last thing you want is a "star network" where a wire must run from every parking space to the main unit. Just imagine the total length! If you can chain them, you only need one wire that runs through all of them. I think RS422 or RS485 will do that.
Even if you use wireless, you have to power them somehow.
RS422 / RS485 can do at least 1000 meters (see the respective wiki articles). Should be sufficient from your description. RS232 can go to approx. 15 meters but might be more susceptible to noise.