How did Renfield knew that Dr Seward wanted to marry Lucy??

From the text, we can infer he garnered his information from the hospital rumor mill. On the other hand, we might believe that he had his information from Dracula.

I thought that this style of conversation might not be pleasant to Mrs. Harker, any more than it was to me, so I joined in, "How did you know I wanted to marry anyone?"

His reply was simply contemptuous, given in a pause in which he turned his eyes from Mrs. Harker to me, instantly turning them back again, "What an asinine question!"

"I don't see that at all, Mr. Renfield," said Mrs. Harker, at once championing me.

He replied to her with as much courtesy and respect as he had shown contempt to me, "You will, of course, understand, Mrs. Harker, that when a man is so loved and honoured as our host is, everything regarding him is of interest in our little community. Dr. Seward is loved not only by his household and his friends, but even by his patients, who, being some of them hardly in mental equilibrium, are apt to distort causes and effects. Since I myself have been an inmate of a lunatic asylum, I cannot but notice that the sophistic tendencies of some of its inmates lean towards the errors of non causa and ignoratio elenche."