My budget is £3k to £3.5k (max) for a share.
Willing to pay circa £50 a month to cover fixed costs.
So, I would guess I would need six people to keep the monthly costs down.
There is currently a Jodel DR1050 for sale in Ireland (looks nice - on Irish register though ex G reg) up for £23k, but it has been up for sale since July 2014 and still has not sold. I personally thought (looking back at previous 1050 for sale ads) that £17k to £20k was more realistic... But I must admit I'm no expert. I notice another one has gone up for sale recently in the UK.
I am in an eight person motor glider syndicate at the moment, and even with 8 people the aircraft is under utilised.
I was therefore comfortable around 6 persons.