[mdjlucan]

How many onion sets can I put into this large pot?

[New shoot]

The more room you give onions, the bigger they get. 

The pot looks to be about the size of a paving slab so 6 sets evenly spaced would give decent results, but if you are not too worried if they are small you could plant more heavily.

   
 

[mdjlucan]

Just looked at my onions some tips are coming out and some are not shall I cut the tips off

[mumofstig]

No, it's their way of telling you they need to be planted -  either into final planting position, or into divided planting trays for now, to plant out later.

[mdjlucan]

They are already planted. They have been in the ground now for a couple of weeks.

[mumofstig]

Then just leave them to grow 

[steven c]

if you have plenty of sets you could plant closer in pot then thin out and use thinnings as spring onions leaving the rest to mature  good luck,