help decide what av receiver under $1k to choose

Less is more!!!  You’d be better off with a great 5.1 system getting a better performing receiver.  Get a good receiver that also does good stereo.  Anthem MRX520 with Paradigm Premier or possibly Prestige Speakers and one of their new Defiance Subs - you’ll do much better than a $1k 7.1 receiver with more speakers of less quality.  A good dealer can show you why that’s the case.

Big +1 @geek101 -- Some speakers are much harder to drive than others, so pick the speakers that appeal to you most and then we can help you choose an AVR that can drive them properly.

Some highly-rated speakers from manufacturers who sell direct offer high value and, very importantly, an in-home trial period. These are worth a good look, especially if you don’t have many good dealers in your area. I’d put them all in the "you almost can’t go wrong" category, especially if you’re not an audiophile and not familiar with the various brands out there. Just a few off the top of my head:

- Aperion Audio - solid speakers and outstanding customer support and in-home trial period
- SVS - also very good with excellent subwoofer options
- Ascend Acoustics - more expensive, but probably the best of these three.

Some good brands you may be able to find at a dealer:

- Monitor Audio
- Paradigm
- B&W

Just some options for you to explore. I’ve built two systems for people with Aperion, and I guarantee you will not be disappointed with their performance. In fact, you’ll probably astonished at how good they sound. And the other two can do the same. Comes down to budget and your own personal tastes in appearance and such. One thing I will stress from personal experience -- pony up for the best center channel speaker you can fit into your budget. 80% of what you hear in home theater comes from the center, so you do not want to skimp there.  I strongly prefer center speakers that are 3-way designs where the tweeter is mounted above a dedicated midrange and flanked on the sides by two woofers.  Among other things, they tend to have better dispersion characteristics so it sounds more uniform as you move off axis, plus the advantage of a dedicated midrange driver that helps with reproducing voices fully and accurately.  Anyway, hope this helps and best of luck.

how many channels you need

7 will be enough.

Zero in on your speakers first

I'm planning to buy some ~$1k receiver (something I found here https://helptochoose.com/best-av-receiver-under-1000). What speakers would you recommend?

as a factory refurb

Are you sure about refurb receivers from Denon?!

Denon X3400 (7ch), which can also be had for cheaper as a factory refurb sold by Accessories4Less. There is also the Marantz 6012 (9ch, expandable to 11ch with external 2ch power amp). Unless you need 13ch, no need to spend more than these, they are already good quality (super linear FR, unclipped 4Vrms pre-outs, >90dB SNR, ~2dB of dynamic power over rated RMS wattage, etc.).

@allinone1989 Zero in on your speakers first and then you can look at what they need. If you want to spend less money then make sure to get efficient speakers i.e speakers that are 8 Ohm minimum and are 96 dB or more efficient. 

Well, a first step is to identify how many channels you need.  That could be 2, 3, 4, 5, 7, 9, etc. and if you want to include ATMOS.  Is your budget just for audio or does it also need to cover a display?

@geek101 it’s so complex. Do you have any basic guides? I promise to read that and will back smarter and ready to answer all questions.

@allinone1989 to get any advice you need specific feature you want like if you want 4k, latest HDMI and what decoding feature do you need, how many speakers need to be powered and what power amplifier output you are looking for, phono stage, latest apps integration, wifi , bluetooth etc etc etc.

In your budget range you can’t go wrong with NAD,Marantz,Cambridge Audio,Outlaw Audio & Harman Kardon...

Best bet is to seek advice on AVSforums.