There is no minimum or maximum amount of texts a fourteen year old child should receive in any given month. This will depend on what the parent sets as an acceptable amount of texts for the child. It also depends on the text plan that the child's phone is set to receive.
There are many other things a 14yr child should be learning instead of just texting. I think a responsible parent should place restrictions on not only tests but the use of the phone in general. Assuming you child does have numerous friends who probably do have use of or have a personal phone -- I would say one or two a day should be sufficient. Your child should have school studies to tend to.
As many as she can afford herself. If you don't like it, then stop paying for it and make her pay for it herself. If you're worried about how many txts she's sending, then maybe that's a sign that she can be doing better thigs with her time. My parents stopped paying for my cell phone when i was thirteen. Being an adult now and living on my own has been easier for me, because i was more independent and self supporting because i payed for everything myself.