Kiara, that is the one thing that Dh and I didn't know, I prayed about it and when I got to the fs they said they putting 4 back (we were only going to put 2), but we also didn't want to risk all chances as a friend of mine did IVF 3 times, first 2 times they put 2 back and it never worked and they then did it 1 last time (due to cash flow) and put 3 and it worked, with all 3.
When we got to the fs room to do the ET, they told us they were putting 4 back because of the grade of our embryo's, we only had 1 grade1 (being the best), and then they rest were 2 grade 2 and 1 grade 3, dh and I were surprised, didn't expect any to take, prayed for at least 1, especially because of the grades.
Look there is a possibilty of multiples which you must be aware of (obviously), the more you put back the more chances you have of more babies, lol. But even if you just put 1 or 2 there is a possibilty of that one splitting so either way.
We actually put 4 back (all 4 took, but the 4th was a B/O.... a bit relieved)