Sorry, wanted to rephrase my reply.

You will be fine with three to four WTFs in an 18x18x24 enclosure. When WTFs are full adults they can get to 5 inches or so in width. This is when you may need to get something more suitable in tank size per frog.
Until then, guessing 6 years or more down the road, your going to have tons of room for many plants and branches to give them some place to play.