As Bruce says, there isn't really very much info. We know they were in the forefront of the Battle of the Pelennor Fields, specifically in the cavalry charge to cover the retreat of the outcompanies and the advance from Minas Tirith after the ride of the Rohirrim. From one of Tolkien's letters, we know that troops looked similar to those on the Bayeux tapestry. The knights would therefore presumably be mounted on un-barded horses and equipped with helmets, chain mail and lances and swords, like Duke William's Norman knights.
The knights would likely be derived from the nobility of Dol Amroth and the surrounding countryside. As nobility are often related, perhaps many of them could have had a (very small) proportion of elvish ancestry as did Prince Imrahil, being descended from Imrazor and Niphrellas (I think!).
As for numbers, we know that a company of mounted knights and a long line of men at arms on foot were provided by Dol Amroth, probably 700-800 in all. A company is generally 100 to 200 men. There would surely have been more troops left in Dol Amroth to protect it from the corsairs, though I'd guess the comment that 'they've only sent a tithe (1/10) of their strength' was inaccurate.
The forces of Dol Amroth fought under the Swan banner, thus Swan knights.
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Rumil of Coedhirion
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