I've used several kinds of root tabs - Seachem Flourish tabs or API tabs are both good.
How many depends on the size of tank, the quantity of plants, and how thickly, or sparsely they are, and how far apart they are from each other.
If the plantings are fairly heavy and quite close together, you could use 1 tab every 4" to 6" or 8" or so. Examples might be say, no more than 3 or 4 tabs needed in a 10 gal tank, whereas a 75 gal tank could likely use 10 to 12, or more - Get the picture ? Push the tabs into the substrate near the heaviest concentration of rootings in the planted areas.
The tabs should be replaced with new about every 4-5 months.
I also use a limited amount of liquid ferts on a semi-weekly basis, (examples are Tetra FloraPride and Flourish comprehensive Supplement), along with Excel, mainly for the benefit of non-rooted Anubias/Java Ferns and the like. Or you could use dry ferts for the water column instead.