Hello,
Here is what I found in docs
There is a 10 MB file upload limit per attachment. However, Business Class and Trello Gold members have a 250 MB file upload limit per attachment. There is no limit on the number of attachments you can have on a card or board. Neither is there an account data storage limit.
I guess there are no limits. I found this info here:
https://help.trello.com/article/769-adding-attachments-to-cards
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