Is iPad Pro 10.5” (2017) only wifi or it needs to be wifi + cellular?
The iPad Pro, regardless of whether it's the 10.5" or 12.9" size, comes in two main configurations - one with Wi-Fi only and one with both Wi-Fi + Cellular.

The reason many of the sellers are pointing out that the model they're selling is Wi-Fi only is because that's the configuration they're selling.

For more detailed information I refer you to the specs provided on Apple's website. You'll see that there are details for both the Wi-Fi models and the Wi-Fi + Cellular models. For example, you'll see differences listed under Size and Weight and so on.