Guys, i don't know about the limit. However, i have just filed 8th year H-1B extension as i my EAD had expired long time back. I'll be working on H1-B receipt.
Just to be on the safe side and it's cheaper, i have also applied for EAD renewal and AP so that i can travel.
Problems with extending H-1B is that if you are working in receipts then you cannot get driver license renewel and if you have an emergency you can't travel. Moreover, even when the petition comes in hand you'll have to apprear for an interivew in your home country to get the stamping if your old stamp has expired more than 1 year ago.
I have learned that no theory works with USCIS and just to save yourself with headaches it's better to apply for EAD/AP>