Luminate Online has no direct cap. Donation Campaigns can be configured to have a cap. If you find and edit the donation campaign, on the second page of the flow is an option to specify a max; this is rarely used. If a donor has tried and is unable to make a large donation, run a decline report (instructions on running a decline report are here) to verify that the problem is not one of the usual causes of transactions failing. If the cause is unclear, contact support who can log into your sites merchant account and see if and what the donation amount cap is.