When entering group booking can you just enter the number of rooms required then split and allocate at a later date?

You do not need to enter the names of all of the guests on a Group Reservation as you are creating it. Once the reservation has been saved, the number of rooms booked will be removed from your inventory immediately and you can allocate the rooms to individual guests once you have been sent a rooming list or, indeed, at a later date (e.g. check-in) if required. 

You can assign room numbers to a group reservation a few different ways. Below are the options: 

Go to Reservations > Group Reservations, click the down arrow next to the GRES number and choose 'Edit Reservation', you will then be redirected to Manage Reservations. Click the + button next to the RES number, which will open up the attached rooms and RESX, here you will see 'assign' under the 'Rooms' column with a down arrow next to them, click this arrow and you will see a list of room numbers to assign the rooms to. 

You can also add in guest names to each room by clicking each of the down arrows under the 'Guests' column.  

Another option is to go from the Group Reservations page and click on the down arrow next to the GRES number and choose 'Edit Group Reservation'. Then when the booking opens up, click on each of the small green '+' icons highlighted on the second screenshot below. This will then open each room (screenshot 3 below), click the down arrows as highlighted and choose your room numbers. Ensure you click 'Update Group Reservation' once you have updated the room numbers to make sure they save (see 4th screenshot below).

Lastly, you can assign rooms to a Group Reservation by going to the Room Schedule, 

,you will see the bookings in 'Unallocated' at the top of the diary view in red. If you click directly on the booking, you can drag and drop them into the room number you wish. Once you have dragged them into the room, ensure you click 'Save' (circled in red on the 2nd screenshot below).