i would like to know how many sensors can be connected to raspberry pi at a time (i would like to connect sensors like temperature sensor,humidity sensor,pir motion sensor and 3 to 4 leds)
This depends on many factors. Main limitation is the number of programmabble GPIO ping you have. In model A and B, there are 17 programmable pins (rest are +5v, 3V3 or GND). On model B+ you have 26 programmable pins (rest are +5V etc.). If you ever run out of pins, you can make a multiplexer which will be able to handle higher amount of devices on the same amount of pins.
TIP: For analog sensors i used MCP3002 Analog-Digital converter. The full wiring takes 5 GPIO pins, 2 power pins and GND.