I thought that if we line up benches in the garden area too, we could probably secure quite a few seats.
Literal
Honorable-garden [gen-の] direction [to-にも] bench [obj-を] line-up [conditional-ば], fairly seat-count [obj-を] secure able [seems-そうだなー] [quotative-と] thought [polite-ました].
座席数 'number of seats' uses the counter 数 for quantity. 確保できそう 'seems like we can secure' stacks 確保 (secure) + できる (can) + そう (seems). なー is the elongated masculine musing particle (lengthened な), giving a contemplative tone. ~と思いました 'I thought' wraps the internal monologue.