I have two betta tanks, a 5 and a 10 gallon, and after buying some plants from someone on here (and having a lot leftover) I'm wondering if there's such a thing as too many plants?
Yes & no.
exhaust exhaust the nutrient supply.
Visually becomes unsightly( personal interpretation)
Plant mass interferes with live stock.
These would be extreme cases.
The no part is, having good plant mass is good for over all tank balance.
If you have enough plant cover, you can try keeping shrimp with your betta's. They can serve as a clean up crew and keep your betta entertained.
If over-planted the CO2 given off by the plants during the night can asphyxiate some fish such as SAEs. This is especially true if one does not discontinue CO2 injection during the dark hours.