Edit: Completely misread things, I thought you had 192.168.10.0 and 192.168.20.0. Yes it would be the whole subnet for a /24, if you wanted less you’d go higher as mentioned.
192.168.0.0/29 would include 192.168.0.1-192.168.0.7.
/28 would get you up to 192.168.0.15.
No, /24 is just 256 IPs.
192.168.0.0/24 = 192.168.0.0 mask 255.255.255.0, or 192.168.0.1-192.168.0.254.
If you want 192.168.0.0 through, say, 192.168.7.254, you need to cover 2k ip’s, which is a /21. The mask would be 255.255.248.0.
A /24 would only be the last octet.