I have an electrum standalone wallet for storing my BTC. I have three questions
1. I read that never store all your coins in one wallet. So wts the solution? should I install two electrum standalone wallets and store my coins in both of them?
If you are keeping your BTC in a cold storage such as a hardware wallet, I don't think you still need to scatter them in different wallets. They're offline devices so they are basically secure enough. However, if you are storing your BTC in hot wallets, storing your BTC in different wallets may add a certain layer of security but it may not be enough in the end to keep it safe.
2. I have written seed on a paper to recover my BTC in case my computer got corrupt. What thing apart from seed should I backup to recover my wallet in case my system is corrupted.
Your seed contains everything that you need in order for you to recover your BTC in case your system is corrupted, lost your phone or laptop, and so on.
3. Strong password is good enough to secure a wallet or I have to do something else also.
The kind of wallet matters. If you are saving your BTC in a hardware wallet, written your seed on a piece of paper and made copies of it stored in different places, created a strong password, and perhaps added an extra safe word for your seed, then everything's good enough. Everything will now boil down to safe-keeping. Make sure no one else has the access to those information and to where they're kept.