in my 195 i have about 10 astrea, 10 trochus, 2 mexican turbos, and about 8 nassarius. i'd like to get rid of about 3 or 4 nassarius.
to answer your question- no. it's not necessary to have tons of snails.
i know people who have 20x the amount of snails i have, in a smaller tank than i have, and have more algae issues than i do.
it's all about nutrients. the more nutrients, the more fuel for algae growth.
if i removed all of my snails right now, it wouldn't make any difference.