What is the connection limit of the proxies?
The dedicated proxies do currently not have a hardcoded limit.
While the premium proxies do not have a connection limit, we limit the number of newly opened connections to 20-30 a second, per server in our network, per-authenticated IP. The exact amount depends on the protocol used (HTTP or Socks) and the server's specifications the individual proxy is running on. For the majority of proxy use-cases, these limitations should be more than sufficient.
The residential proxies do not have a hard-coded limit. Your only limitation is the bandwidth allocated to your account.