For a one-syllable word, you make the comparative with -er:
scarcer
For a word of three or more syllables, you make the comparative with the word "more":
more abundant
For two-syllable words, "more" is used more often, but there are many exceptions, and there are words where either form is accepted.
You posted this seven hours ago. I hope next time we'll give you a faster answer, prompter, more rapid, more immediate.
regards
edward