✅ Neutral / Formal (best for IELTS Task 2)
numerous
a large number of
a considerable number of
a significant number of
a substantial amount of
countless
an abundance of
a great deal of
a wide range of
a variety of

✅ Academic / Advanced
a plethora of
a multitude of
an array of
an extensive number of
a host of
copious amounts of
an overwhelming number of