How much more data does my classifier need?
I think you already have enough data. Your problem seems to be one of generalization, which is in general a tricky one to solve.
For SVM, depending on what kind you have, you could try tinkering with the parameters and kernels.
Have a look here. -- https://stats.stackexchange.com/questions/35276/svm-overfitting-curse-of-dimensionality?utm_medium=organic&amp;utm_source=google_rich_qa&amp;utm_campaign=google_rich_qa