You haven't said what how you are using the mosfet, so it's difficult to accurately answer your question. As Tony points out, there will usually be some other parameter that changes along the way. You need to know what affect other parameters may have on your circuit.
But if increased current capacity is your only goal, there is a big trap you need to look out for.
One important fact that everyone need to be aware of when choosing mosfets, is that the current rating has a fine print disclaimer that causes problems if not taken into account.
You will notice on the "Continuous Drain Current" line of the IRFP450 data sheet if gives two current ratings. The 14 amp rating is at Tc=25C. Tc is case temperature, not ambient. So it will only handle 14 amps continuous if the case is kept at 25C all of the time. Not as easy as it sounds.
Let's look at a few items from the datasheet.
1) The Rdson number is 0.4 ohms at 10Vgs. So when carrying 14 amps, it will be dissipating about 78 watts of heat. So, it will only carry 14 amps if you can keep the case at 25C while it is pushing out 78 watts of heat. There is no passive heatsink/fan combination that can do that unless you're running it in a freezer. Otherwise you'll need to resort to cold water or refrigerated cooling.
2) Junction temperature. The sum of the junction to case and the case to sink resistances is 0.89 Degrees per watt. This means that the the junction is .89 degrees hotter than the case
for each watt of power. So, at 78 watts, the junction is 69 degrees hotter than the case. If the case is at 25C, the junction is at 94C. Lets take that number down to the graph at Figure 4. That shows us that our Rdson is really about 75% higher than we thought. We need to recalculate everything again based on the new info. etc, etc, etc.
It comes down to the fact that the current rating of most mosfets is a calculated number as to what current will put the junction temperature
at it's failure point when the case is
at 25C. Doable in a cryogenics lab, but not really a practical value for most real world applications.
So, if you're looking for a mosfet to continuously carry 20 amps, you will probably need a mosfet(s) with current capacity 3 to 10 times 20 amps. A project I am doing right now needs to switch 60Amps DC. I am using four 100 amp mosfets. Maybe a little overkill but, other than the copper plates that carry the current, there is no heatsink.
This is the datasheet I used for this discussion.
https://www.vishay.com/docs/91233/91233.pdf