To get the most out of yucca you really need to make threads of around 10lb breaking strain. Then work out the required strength. 50lbs bow = 200lbs string + 10lbs extra for a little safety, 210/10 = 21 strands. 21/3 = 7. 7 strands per ply (3ply string).
If you just make a 2 or 3 ply string then it will be fairly large diameter-wise and prone to breaking.
As already mention Tim Bakers excellent chapter in TTB2 is an absolute must for anyone wanting to make quality strings and not something just thrown together.
3 ply is better than 2 ply because the outer fibers are less strained so the string can be lighter or safer (your choice). It is also more tubular with less dead mass and therefore less likely to be 'worn out' prematurely. It isn't much harder when you've made a few.
Try making a string like i've described above and a 2 ply string of the same breaking strain. You will find the simple 2 ply to be heavier,less easy to knot, less durable and not as nice to look at.
My ltest yucca string has been shot at least a few hundred times with a 60lb recurve (lot of string tension), I was a little worried to begin(because it's a really nice bow!) with but it's settled down nicely now and as long as it's kept waxed so it doesn't dry out I have faith in it.
Making a real quality string from plant to shooting is a very satisfing thing. Good luck