The Conference of Presidents decides on the allocation of seats among the political groups and the non-attached members. In addition seats are needed for the Commission and Council representatives.
The authorising provision is Rule 32 of the European Parliament’s Rules of Procedure (16th edition, October 2008):
Rule 32 Allocation of seats in the Chamber
The Conference of Presidents shall decide how seats in the Chamber are to be allocated among the political groups, the non-attached Members and the institutions of the European Union.
I have tried to test the accessibility of information about the internal life of the European Parliament through its own web pages.
I managed to find a European Parliament web page promising a link to the seating arrangements, but it turned out to be an error page.
The customary lack of accessible information about the decisions of internal bodies of the EP continued with regard to the Conference of Presidents in general and concerning the allocation of seats.