Yes satan can invade our dreams. God also gives dreams, but remember God isn't the author of confusion. Pray over it for understanding in Jesus name. Also pray over your sleep and pray that what God promised you, which is sweet sleep that you will have and bind up night terror ,witchcraft, divination in Jesus name.
My daughter used to get really bad nightmares and this is what I had to do and it went away. I also had to get everything out of my house that would open the door to it.
About your dream and the number 20 being missing.
The number 20 is represented in Hebrew by the letter caph, in form of opened hand, to seize and hold.
After looking at the biblical meanings of all the numbers from 1-20
The only thing that clicked is that 20 means in form of open hand, seize and hold.
This scripture came to mind> And from the days of John the Baptist until now the kingdom of heaven suffereth violence, and the violent take it by force.
And also Ephesians 6 about our armor and standing.
You see all these numbers in Hebrew represent something we don't want in our life, such as the things listed under the curses in Duet 28, and also the things we are supposed to have according to the blessings of God.
Take a look>> http://www.christian-resources-today.com/biblical-... (it's not a bad site)
So with 20 being missing, it makes me think that maybe you need to practice/ learn spiritual warfare, by praying, fasting, taking authority over the things that's happening that is against God's will for you in Jesus name.
Repent & take back the territory the devil has gained and stand in faith, trusting and believing God and fight the good fight.
One important line of defense is tithing- there's a promise on our tithe- The devour is rebuked for our sakes because we are tithers.
I have many pages on prayers of deliverance, warfare, authority and faith if your interested. Here's some of them>>
I hope this helps:)
God bless you!