You can truly use both as you want. example : il y a plein de choses que j'aime faire. il y a beaucoup de choses que j'aime faire. The both are totally correct ! But you have to be careful because "plein" can means "beaucoup", and in that case you can just use the one you want. But "plein" can also means "rempli " and in that case you cannot use "beaucoup". example : Il parlait avec les yeux pleins de larmes. J'ai toujours connu leur maison pleine d'amis.