Do we know which chapel Royal they want to use? There is one opening up to a courtyard of the Palace, which is the main seat of the "Chapel Royal" as the religious body, consisting of the Dean and Sub-Dean and other priests of the Royal Household. On the other side of Marlborough Road there is another which is called The Queen's Chapel... Both are chapels Royals - used for the personal services of the Royal family, just like St. George's Chapel in Windsor and some others.
That's what Wiki has to say:
The chapel at St James's has been used regularly since 1702 and is the most commonly used facility today. Located in the main block of
St James's Palace, it was built c. 1540 and altered since, most notably by Sir
Robert Smirke in 1837. The large window to the right of the palace gatehouse is in the north wall of this chapel which is laid out on a north-south rather than the usual east-west axis. Its ceiling richly decorated with royal initials and coats of arms is said to have been painted by
Holbein.
The separate
Queen's Chapel, once also part of the St James's Palace compound, was built between 1623 and 1625 as a Roman Catholic chapel, at a time when the construction of Catholic churches was prohibited in England, for Queen
Henrietta Maria, wife of
Charles I. From the 1690s it was used by Continental Protestant courtiers and became known as the German chapel. After the adjacent apartments burnt down in 1809 they were not replaced, and in 1856–57 Marlborough Road was built between the palace and Queen's Chapel. (End of quote)
Plus there is The Queen's Chapel at the Savoy (on the Savoy Hill Estate, part of the former Savoy Palace, that belongs to the Duchy of Lancaster) which is also known as a chapel Royal of London, as it services the queen as The Duke of Lancaster and is the home of the Royal Victorian Order. It's chaplain is part of the team of the priests of the Chapel Royal (which has the same name but is, as I said, the religious body that cares for HM's and the RF's religious needs.)
So we first need to know which church is going to host the wedding and then we could look up the number of guests.