For each Zyte Smart Proxy Manager(formerly Crawlera) plan there is a limit to Concurrency (Requests that can be made parallel at a time). Concurrency is cumulative of all domains crawled at a time and also requests through different Smart Proxy Manager accounts in the Organization. 

Hence when the concurrency is more than the limit of the plan, Smart Proxy Manager gives the error message of 429 (Parallel Connection limit reached). You would need to lower the concurrency or upgrade to higher plans to have higher concurrency.