You can use Splice on as many devices as you like—there's no limit!
Please note, if you download Splice on a new device, you'll need to re-download your library to use your samples. You can follow the steps outlined here to do this.
While it's fine to use Splice on multiple devices, please remember that your Splice account is meant for your use only. Sharing accounts is not allowed, and each account should only be used by the account holder.