Thank you for writing to us.
As per 200.7.x Release Notes, there are 2 conditions to be met to create 50 APs Unleashed Network
1) All the APs must be 802.11ac Wave 2
2) Master must not be in Gateway mode.
However, in this case R310 is 80211ac Wave 1 AP and hence, only 25 APs Unleashed network can be set up.
Hope this helps.