수많은 means "numerous, a lot of, many" 수많은 is composed of 수많(다) + 은 수많다 means "There are large number of something" 은 is a determiner particle. 수많은 사람들 means "numerous people" 있었네 is composed of 있(다)+었+네 었 is a past tense particle 네 is an ending particle used for a just realized fact with a nuance of exclamation. 있었네(요) and 있었어(요) have same meaning but 있었네(요) has a nuance of detection and exclamation.

수많다 means "a lot, countless, numerous" and 사람 means "people". So, it means "a lot of people" = more than "many". And the verb ending -네 means you're saying it with some emotion or surprise. Like "wow!". 있었네(요) means "wow, there were...". So... "(wow,) There were a lot of people inside!".