How many credit card should somebody have? Any hacks to help one rmb when to pay their bills for credit cards?

It really depends if one can handle and remember. I used to have at least 1 to 2 cards from all the banks, total more than 30 cards for the last 10 years! :P It's a mess to remember the bills to pay, when will they bill the annual fees etc.

The reason for so many is because I like to get every possible deals possible whenver possible. End up very messy and hard to maintain.

On the other hand, 1 or 2 cards helps you to focus your strategry of Cashback or Miles. As each games / camp has it's strategy of min spending and cconditions to follow. Seach seedly blog and seedly group facebook on how to hack each games or feel free to ask me more :)

Answer to your question:

So the number of cards one should own : 1 to 2 or As much as you want!

(haha .. i know it seems like I'm not answering your questions, because to me is the same)

Alternative way: Each bank, login to their iBanking, setup the reminder via email and sms so they will remind you when bill comes and you just pay each bank/card when you receive each reminder.
Only 1 rule to follow to avoid late payment penalties:

After you spend, just use your phone banking app to pay the credit card the amount you spend or a bit more immediately, or the full amount if you know roughly how much you will spend on the card monthly. This will help avoid forgetting bills payment / overdue interest and late payment penalties. Annual Fee charges ? How ?

As annul fee comes, you can see in your bill and always call the bank to waive, if they don't just tell the hotline officer nicely that you really like that card and would be nice if they can help to waive the annual fee. If they are not willing, then just cancel the card. So no annual fees will be leavied on you.
Thanks for reading so much for this long answer, if you stil wish to know more, read on for the Bonus: :p

Up Your Game Hack:

Play the game to get new card gift / cashback deals hack:

This is my personal secret methods that i use, I'm gonna share it for the first time...drum rolll please :P hahahahaha

Use your smartphone use the note apps or any of the note apps on App / Play store and create a page where you can lock, write down all your credit card details, especially the card activation date, cancellations date. This is to make sure you know what you have, and actively cancel these cards when you no longer need them, and note down the cancellations date for the later game. Whenever you see online be it on Singsaver, Gobear or on the street the banks promoting their cards, if you had it before, if you had it more than 1 year from the cancellation date, you'll be qualify for the higher tier welcomes gifts such as 80 / 120 cash back (except if the terms says "new to bank" then we'll not qualify). With that you can repeat this cycle to get freebies from the bank for applying their card! :)
Finally:

I know it seems like a lot of work to spend and pay immediately. To save the effort, you just pay the total amount you estimate you'll spend on the card monthly in advanced.

If you didn't spend that much , your next month bill is prepaid in advance! So just adjust back next month and deploy the same strategy! There you go, hope this sharing benefits you! :) Cheers!

I check with credit card company about the billing cycle for my credit card and set a reminder on my mobile with Google calendar.
I have a bank account and credit card under the same bank, so that I could set GIRO payment within the bank.
However, my bank always send me a SMS reminder 1 week before payment due date.

I have 2 - one from OCBC and one from Citibank. I only use the Citibank one however for the rewards points but I don't really like to keep track of so many bills so just left the Ocbc card aside.

For me I got 6 credit card. 3 for regular use. 3 just register to get welcome gift last time (soon will cancel). I can't remember the payment date either. So I use auto giro every month. Just make sure don't overspend than what you have.