If you already decide sand will be the substrate then buy 1 bag ,I believed it is 50 lbs.That is enough for just 1 inch thick lay.If you think you need thicker than buy 2 bags.Some bag come with lots of dirty sand dust.Good luck and wash them well
I would go with 2 bags, you may not need 2 whole bags right away. You want about 2.5" of substrate for planting. Rinse the first bag and dump it in, on the second bag don't rinse the whole bag at once, rinse enough to get the depth you want keep the rest dry to store for latter use. As time goes on you will loose sand here and there, so you will have some replacement sand already.
Yes like max said I have a 29 gallon that I put 1 inch layer of pool filter sand and let me tell you plants will not stay down I had to add another half bucket of it to get them to stay. And I am about to rescape tomorrow and up it more so plants stay down better