how many 7400 can the 74LS04 safely drive
This seems like a bit of a trick question. If both the 7400 inputs are tied together, the total current draw when low will still be 1.6mA, as should be obvious from the schematic below.  



The same can not be said of many other gates such as, say, 7402. 



So you can drive any combination of individual inputs or pairs of inputs on the same gate that add up to 5 total. Trivially, if you hold the other input low the gate does not draw any current (nor will the output change). I don't think your question is stated unambiguously enough to give a numeric answer, but if you read the problem statement carefully you should be able to figure it out with this major clue.