If I do not have enough AV ports on my TV, can I use a Roku by plugging it into the Input ports of my DVD player?

No. The ports on your DVD player are outputs, not inputs.

You can buy a AV switch for your TV, and connect everything to that. These switches are sold in any store that sells TVs. AV switch is about $20. HDMI switch is about $40.

Most DVD players do not have any AV input ports! DVRs (digital video recorders) from regions outside the USA sometimes have an AV input or AV passthru port, but you would need to check with the manual.

What may work is if you have a number of devices that are not "always on", you can use a cheap HDMI 2/3 into 1 switch. Not being "always on" is important as these have no remote and autoswitch depending on thich device is supplying power via it's HDMI output.