There are surely no strict limits on polygons count for perfromance - you will need to do tests, but generally, from what I see other developers achieve in Defold - it is possible! Defold Scene 3D by @aglitchman is running on mobile on HMTL5 smoothly for me - you can use the project for testing as it’s open source 
Also today I have seen amazing screenshots in this tread on Frustum Culling which as a feature is currently only available for sprites in Defold, but if it will be available for models and meshes - Imagine what you can do!
(I don’t know if those are screenshots from Defold for sure - are they @morgerion ?)
P.S. Aboslutely amazing! 