I believe you need to use custom dialplan, as you need to use the Transfer application (before the call is answered). I think the required Transfer application parameters are:
Transfer(PJSIP/sip:<user>@<domain>)
where user would normally be the redirected phone number and domain would normally be that from the server setting.
Limits on the number of simultaneous calls are a commercial constraint; SIP doesn’t limit the number, and it is only the underlying IP bandwidth that is limited. Many providers won’t accept redirects, and, at least in theory, they could include redirected calls in the the limit on simultaneous calls.