Total records linked to another table

This should be easy. Create a field (numeric) on the events table known as Capacity. This will be constant for that event.

Then create a new summary field based on the Event-< Registration relationship known as total number of registrations.

Then create a formula numeric field in the events table known as free seats which is [Capacity] - [Total number of Registrations].

Hi Julie,

A) You can in your event table you can have a Total Positions field with an integer value denoting the total capacity of participants for the event.
B) You can then on your table for participants have a hidden formula integer field (called say [Used Position]) with the number "1" set as the value in the formula.
C) Go to table to table relationships under your event table, under the master table (events) click add summary field and summarize the hidden integer formula field. call this [Total Used Positions] This will provide the total count of participants assigned to each task. 
D) Create a new field called [Remaining Position] in the events table integer formula and put in the formula: [Total Positions]-[Total Used Positions]
E) You can further limit the ability to create records once the positions are used up on each task but this requires a bit more work/scripting.

hope this helps..

Josh

Thank you. Both were helpful!