Well, you'd want your tin to be about 2/3 full before baking. So why not get a jug of water, and pouring the water into the #5 tin, cup by cup. When you reach 2/3 full, you will know how many cups of batter you would need.
Thanks. Your input is very useful!
I looked at the website you listed and was surprised to read that the pan was not leak-proof and you should use a liner. I assume you were aware of that at the time of purchase. Anyway, the only number pan I have used was the Wilton #1 and I used about 6 cups of batter in it. If I'm not sure, I usually fill about 2/3 full and then put a pan on the oven rack below the rack the cake is on just in case it runs over. Hope that helps. Best of luck.
Hi, I made a #5 b-day cake this past november and it turned out great. I used 4 boxes. Fill it 2/3. I didn't buy the pan, I used a 11x5 square pan and did a 2 layer cake. You can even do 3 layers if you like. I filled it, reg/fre' it then carve it. It's up to you which way to go. I bought a football pan and let me telly I'd rather carve the damn thing. Sometimes it's just not worth the $ and time spent on it. hope this helps