It depends on what you mean by atlas, if you are not using virtual textures then the limit is 8192 x 8192 in which you can fit 4x 4k texture or 16x 2k textures or 64 1k textures inside and so on and so forth,
If you are using virtual texture then the limit is very higher for context I did a 116x 4k textures into 1 large textures ( they are called UDIMs generally), I made a custom plugin and released on fab for UE which allows creation, modification, extraction of UDIM textures & many more features inside the engine.
Link: UDIM Editor | Fab