On UCx systems, 10 licenses allow you to configure up to 10 extensions. Each of these extensions can use all features available on the system (to name a few - voicemail, personal MeetMe conference room, automatic/manual call recording, NAT traversal on Nortel phones, Find Me Follow Me, membership in ring groups, page groups, Call Center agent functions and so on).
Devices that count as extensions are SIP phones, Nortel IP phones (i2000 series, 1100 series and 1200 series), analog phones (provided via a SIP gateway or UCx Digital Gateway) and Nortel digital phones (provided via UCx Digital Gateway).
SIP trunks are a basic built-in capability of UCx systems. Any number of SIP trunks - and any number of channels on these trunks - can be used without any additional license.
When you get a SIP trunk from a provider in your area or some global provider, you just configure the SIP trunk in the management interface and define outbound route(s) that should use this trunk. After that, all extension that are allowed to use the route(s) can then use the trunk. Clearly, you can have trunks from multiple SIP trunk providers at the same time.
For PSTN access, you have to use SIP gateways. E-MetroTel offers several types of analog and ISDN gateways, but you can use SIP gateways from vendors they don't carry, too.
These gateways are connected to the UCx system using SIP trunks. Once you set them up, there is no real difference (from configuration perspective) between a SIP trunk from a provider and a SIP trunk for the gateway.
If you need more trunks, it can be extremely simple if you use a SIP trunk provider. Many SIP trunk providers allow you to increase the maximum number of channels based on your immediate needs (for example seasonal call volume increase or a one shot campaign you may want to run) and then reduce them back. For example, you can check out Skype - they provide SIP trunks with a lot of flexibility.
If you need additional PSTN trunks, you'd have to add another gateway. BTW - for companies with several locations, it is possible to use the local gateway as the preferred route and a gateway from another location as an overflow route. If there is network connectivity in place, it's just matter of setting it up (and making sure the required network bandwidth is available

.