I really do recommend starting with as many plants as you can. It helps to prevent problems such as algae. Also, you save money on shipping if you buy at once, as it often costs as much to ship 5 plants as it does to ship 12. If you only want low light plants, a 125 watt CFL bulb over a 10 gallon tank should be fine as long as you keep it raised above the tank a few inches. You'd probably need to fertilize them though.
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