I have 3 x 8w t5 in the hood of my 14x14x12 tank and 1 is enough with no co2. 2 is enough with co2. And if i switch all three on i grow lots of algae, very quickly. I think bertie has a tank similar size to yours and he had to use one tube for a while while he got his co2 right. Have a look through his posts. Beginner plants generally mean low light, low co2 and 1 tube in hood will grow these quite well. I have a friend who grows beginner plants under 1x t8, in a tank the same as your planning, when he remembers nutrients they do quite well. 2x t5 will be ok as long as you are able to only use one just in case you need to