It gives me a Merge error but when i select 140 targets it works ok so i gues thats the magic number for targets to handle on one dataset or does it works on   a megabyte size? 

What's the error you are seeing?

Also the recommended limit to the number of Trackables per dataset is 60, though more is possible. So I'd recommend experimenting with smaller DataSets to observe their performance.

Your use case is well suited to our upcoming Vuforia Web Service - https://developer.qualcomm.com/mobile-development/mobile-technologies/augmented-reality