As Richard mentions, the current maximum is 50, but they intend to increase the maximum to 100 at some point:
Just to point out, that the minimum investment for a company within a pie is 0.20 GBP, therefore if you make a slice in your pie too small the minimum investment you can make into the pie will increase.
Eg. For a GBP user: If you have 0.50% (current minimum) allocated to a stock within a pie, considering the mimimum investment of 0.20 GBP the minimum investment you can make into the pie would be 40 GBP, to ensure that all companies/ETFs get allocated at least 0.20 GBP.