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5%.
From the UEFA CL regs:-
"
Ticket allocations14.06 All clubs in the competition must make at least 5% of the total capacity of their stadium available exclusively to visiting supporters, in a segregated, safe area. In addition, visiting clubs are entitled to purchase up to 200 top-category tickets (unless otherwise agreed between the two clubs in question) for their VIP supporters, sponsors, etc. (see Articles 16 and 23 of the UEFA Stadium Infrastructure Regulations and Article 19 of the UEFA Safety and Security Regulations).
"
Surely there's a minimum allocation for CL like there is for FA Cup games?