10,000 characters for 20 marriages = 500 characters per entry, on average. David are you sure this is what is required?
I've no idea of your formatting or how much information is contained in each entry, but using the following as an example...
Elizabeth II (b 21 April 1926), elder dau of George VI & Elizabeth née Bowes-Lyon, m 20 November 1947
Philip (b 10 June 1921), son of Prince Andrew of Greece & Princess Alice née Battenberg.
= just under 200 characters, including spaces.
If you would be doing a cut and paste from your original database directly into each post, from an Admin and Forums point of view the number of posts required isn't an issue. However, if you have to re-type or re-format everything then I understand it is a great deal of work indeed.
For interest's sake, would you be able to post just one marriage entry as an example to give us all an idea of how it looks?