I have a couple questions on making this cake. (Not my cake)

1) How much batter do you guys typically use for 1/2 sheet pan.

2) Does anyone have any tips on how to make this?

For the mermaid tail scales you can use this method. 

According to the Wilton serving chart a 12-18 sheet would take 16 cups of batter. 

if the entire thing is from one sheet then you can just bake they sheet then cut out the design? It looks basic enough

how to cook that has a video showing that sort of pattern 

Depending on what size that 1/2 sheet pan is it can take 2 cake mixes.   I *NEVER!* measure batter.......it is sooo messy and not the least bit accurate.  Instead fill the pan 2/3rds full.  If the pan is 2" deep then the batter should be 1& 1/4 to 1&1/2" deep. 

Another way to make those scales is to use tip 104 (or a larger size of the same rose petal type) and pipe upside down "U"s