The fuller context of the story is required to answer this (from chapter 9):
I believe Van Helsing saw it, too, for I saw the quick look under his bushy brows that I knew of old. Then he began to chat of all things except ourselves and diseases and with such an infinite geniality that I could see poor Lucy’s pretense of animation merge into reality. Then, without any seeming change, he brought the conversation gently round to his visit, and suavely said,
“My dear young miss, I have the so great pleasure because you are so much beloved. That is much, my dear, even were there that which I do not see. They told me you were down in the spirit, and that you were of a ghastly pale. To them I say ‘Pouf!’”
From this, it is clear that Van Helsing (the doctor) has been called out by people who have said that Lucy was ‘down in the spirit’ and that she had a pallour and illness about her. That pallour and illness is “that which I do not see”; in other words, he is saying (in order to comfort her) that he can see no pallour or illness on her.
“That is much” simply means that the previous statement (that Lucy is much beloved) is worth a lot.
To paraphrase the whole thing into more modern and explanatory tones:
“My dear girl, this is such a great pleasure for me because people love you so much. And that would be worth a lot, even if that illness they told me about had been there (but of course I see no indication that it is).