Islam encourages us to have multiple children. One hadith states that if you have three daughters and you raise them then you deserve to fo to Heaven.
It’s not haram to have one child only. Allah won’t be angry with you. But you will miss out on greater blessings. If you raise more children who are righteous, then you are eternally investing for yourself. Long after we are gone from this world, we receive amazing rewards for the good deeds that our children and grandchildren do.