How many is "too many"? I plan on closing the account before purchasing a PPOR, but may purchase one within 12 months.
Unfortunately this is up to the bank's discretion. I've heard stories of people who were declined for loans because of credit cards (this couple had a $100k limit between them) but I've also heard of a bank not caring after my friend explained that he only got credit cards for the rewards points. It's hard to tell. And yes, it absolutely affects your credit scores.
I applied and was approved for the CommBank card you mentioned. I already have other cards and only intend to hold this one until I get my $250. Given this, I lowered my limit to $1000 although they offered me a much higher amount. If you did this, I personally would go for it. I wouldn't think $1,000 would make much of a difference given the rest of your finances are solid.
There are heaps of great credit card offers all the time. Here are some of my favourites:
Citibank Signature - no annual fee, generous limit, and the ability to take 80% of your credit limit as cash, borrowed at 0%. I put this money straight onto my offset account, saving me more interest than I could earn with a savings account. I then pay the balance back in full when it comes due. I believe the balance transfer term started at 6 months when I got it. 2 years and two extensions later, I'm still on the 0%.
28 Degrees Mastercard - no annual fee. This is my go-to card for travelling and overseas purchases because it has no foreign transaction fees or currency conversion fees. It also has price protection, which means that if you buy something in Australia which then drops in price from the same store within 6 months, they'll refund you the difference. Price protection is free IF you have the discipline to pay your card off in full BEFORE the statement comes out (which, as a YNAB user, you shouldn't have an issue with).
Westpac Altitude Black - This is a good candidate for a credit card churn (ie a card that you only keep for the year). I think I got 80,000 points when I first applied for it, plus a great rate for actually earning frequent flyer points. Comes with an Amex and a Mastercard. First year is free-- so get in, use it, and then close it before you have to pay the $400 annual fee!