First: it is not possible to run more than one BGP prozess on the same router. But it is possible to run BGP sessions with different local as number within the same BGP process: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-s/irg-15-s-book/irg-dual-as.html
If you only want to announce a route with a different AS number, you may "preload" the AS-Path with another AS. Just use "set as-path prepend" within a route-map to announce some prefixes with a "preloaded as-path" (with Cisco IOS, the "preloaded path" may include other AS numbers as the locally configured AS of the BGP process).
Master Jedi is correct; " it is not possible to run more than one BGP instance" so it is not possible to have multiple Public AS numbers on same router. There is a replaced AS # with another one command in BGP for purpose of changing (merging) from one AS to another. see https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-15-mt-book/irg-remove-as.html
Also , Local - as number might help you, see https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-15-mt-book/irg-local-as_policy.htm…