I have a client who uses PayPal Plus, PayPal Plus OTO, Order Summary, and Multiple Bumps on his funnels.

He has asked me how many Bumps we can add.
There's not a limit built into any of the add ons.

But there are some limitations with ClickFunnels.  The maximum length of the order description sent to the payment processor is 500 characters.  Above that it will throw and error when trying to submit the order.  
The order description is generated by combining all the product names together in a comma delimited list.  
So, if you have a lot of products with long names on a particular order, that could cause an error.

From a practical strategy point, I would not recommend having more than 3 or 4 bumps.  The more you have the less effective they will become.  Having more than one can boost profitability, but having too many can have the opposite effect.