Are you talking about filling up the entire center section of this? I'm gonna say 'a whole bunch', like just buy the biggest bottle they have and see how much it fills it up. You can always add more later and it doesn't hurt to have extra laying around so you can use what's in there to seed another tank and refill it.
If there's one thing you'll learn about this hobby it's that you really can't have too much stuff or too many extras. You're going to accumulate a lot of stuff, a little bit of extra matrix is just the beginning. Eventually you'll have more heaters and pumps laying around than you know what to do with.