Elixir runs on the same virtual machine as Erlang, and it's thus subject to the same atom limits as Erlang.
You can check the current limit with :erlang.system_info(:atom_limit) , and you can change the limit by passing the +t flag to the Erlang virtual machine, using --erl to let the flag through to Erlang:

$ elixir -e 'IO.inspect :erlang.system_info(:atom_limit)'
1048576
$ elixir --erl "+t 2000000" -e 'IO.inspect :erlang.system_info(:atom_limit)'
2000000

However, if you find yourself running out of atoms, you should probably try solving the problem in another way.