Practise counting in 5s. You can count using fingers in groups of 5: 2 hands is 10 fingers. Try adding someone else's hands so that you have 4 hands. Count the fingers in 5s: 5, 10, 15, 20. You could try counting higher if you can.
You could also put your toys, bricks, spoons, forks, coins, counters etc into groups of 5 and practise counting them.