The number of circuits that can be on a 100 amp electrical panel varies depending on the manufacturer. If you have access to a 100 circuit panel there is no stopping you from reaching that capacity as long as you are not exceeding the load of the service which is determined by doing a load calculation for your home.
More times than not 100 amp panels are more than enough to supply power to the home. To determine if you require more than this a house load calculation will need to be done.
The difference between a 100 amp and 200 amp electrical panel is the size of the main breaker, the rating of the busbar in the panel, the rating of the panel enclosure, and the size of the wire supplying power to your home. A 200 amp panel can supply more power to your devices but a 100 amp panel is enough to do the job required.
To replace a 100 amp panel the cost depends on whether or not the panel is a subpanel or if it is a main electrical panel. Subpanels cost significantly less to replace than the main electrical panel due to requiring power disconnection.