There is no limit for delivering blood herbs to Wyda because there are two rewards related to this Mini World Change.
When you give Wyda the blood herbs, she gives you a witchesbroom as reward, and she can rarely give you a "Torn Teddy". When you get the Torn Teddy, you will also get the Achievement "Torn Treasures".
That's why blood herbs are good profit on all worlds. Not only they are needed for the Item + Achievement, but they are also used as decoration, and since it's not guaranteed to give her a blood herb and get the achievement, the number of times you can give her a blood herb is unlimited, because there are players who also like to get more than 1 Torn Teddy.
All you need to do, just like the other answer said, is to say "blood herb" and "yes", so she will accept it from you.