Sites and the security services they use keep scores for addresses. The score draws on the kind of network, how many requests the address has sent, whether it appeared in abuse reports, and whether it has been seen acting like a bot.
A low score does not always mean a block. More often the visitor gets captchas, slower answers or a login check. Reputation is why the type of proxy matters more than the count: one address on a home network is trusted more than a hundred in a hosting block.
Read next: Residential vs datacenter vs mobile proxies