There isn't a limit on the number transactions to enable mining. Mining is always on by default. It starts mining with 0 tx, but will never find a block, so transactions must be brought in. You can mine with 1 tx or 10k tx or 50k tx. Once you have established your transactions for mining, you have them for life. You don't need to do it again unless you want to get more tx hashes to feed more CPU to do more PoW to increase your hash power.