Uploading data into the CRM I've built - Not working!

So I’ve finally finished building a CRM for our business. Throughout the building process I’ve tested uploading a few rows of data in as well as single manual new data creates - All have worked fine.

I’ve now tried to upload a database of 500 rows and around 10 columns and keep getting the popup;

" This list is too long to be processed. "

I don’t mind paying for a way to upload the data, what plugin would be needed and how would that be done?

Also we are hoping the resell this on - Can anyone suggest what we would need so customers wouldn’t have this problem?