SN7400: SN7400

Suganya,

Please refer to these FAQs

[FAQ] How do I determine the output voltage (VOH, VOL) or output current (IOH, IOL) of a CMOS logic device?

Specifically part 2 and

[FAQ] What method is best used for estimating specification values between those given in the datasheet?

For these you'll also want to look at the values from the electrical characteristics table. This will show how much current the device can output while maintaining the VOH and VOL values listed. For example from the SN74LS240 : 

The minimum leakage values aren't listed since they're so small that it wouldn't impact anything. That would be absolute best case however. I wouldn't recommend designing around that.

Thanks,
Rami