OK, after some head-bashing I've realised there's a Linux-specific (Android being based on Linux) mistake being made here - when you type /src/dir the initial / implies that you're looking in the filesystem root, mounted at /. In this case, however, you're probably referring to the current directory, which will likely be in /sdcard/ since you are not rooted. Simply omit the first slash and you're good to go (personally tested).
find src/dir -type f -exec mv --backup=numbered -t dst/dir {} +
If you find still can't perform the function (especially due to an error such as unrecognized option, make sure you install the latest Linux version instead of the one built in to your kernel. This can be done in Termux using pkg install findutils