As to your confusion about the module's name, Shared Port Adapter, in addition to what Giuseppe describes, you might find this (old thread) https://community.cisco.com/t5/routing/what-is-spa-share-port-adaptor-and-sip/td-p/2376181 of interest.
As to your question of available bandwidth, when adding module ports, in general, besides licensing restrictions, module interfaces have bandwidth restrictions, and possibly further bandwidth restrictions due to other hardware limitations of the platform, beyond the module being used, itself.
Sometimes a module will have more ports, with their aggregate bandwidth, that exceeds the module's bandwidth (your question). Or, sometimes, modules (and line cards) will "divide" the available aggregate bandwidth between different sets of ports (sometimes in "interesting" ways.)
For example of an additional module limitation, specific to your SPA-5X1GE-V, if you search for "PRE2" in https://www.cisco.com/c/en/us/td/docs/interfaces_modules/shared_port_adapters/install_upgrade/uBR10012/b_10k_sip_spa_hig/b_10k_sip_spa_hig_chapter_011.pdf, you'll find a PRE2 (Performance Routing Engine) in that platform limits the SPA-5X1GE-V2 to 2 gig. (NB: The PREs appear to be modules used in the uBR100xx/ESR100xx routers.)
Lastly, it's also sometimes possible to oversubscribe bandwidth capacity (i.e. hardware or software license) on the whole platform when adding ports.
