Data pads

Were you looking for Data pad, the in-universe piece of technology? The Data pads are hidden computer pads encountered throughout the Halo: Reach campaign. Similar to the Terminals found in Halo 3 and the Audio logs found in Halo 3: ODST, the pads contain their own, self-contained story, presented as exchanges between members of a mysterious AI collective known as the Assembly.

A total of 19 data pads can be found at various locations in the campaign, with at least two data pads in every campaign level. Each data pad is numbered; letters or words are replaced with numbers in each data pad message representing the number of the data pad. Data pads numbered 1-9 can be found on any difficulty settings whereas data pads 10-19 can only be found on the Legendary difficulty settings.

Trivia
If you multiply the 15 seconds it would take to glass 1 acre with the 130 billion acres of Earth(also known in science world as 1.3*10^11) = 1.95*10^12 Seconds for ONE ship to glass earth.
 * From Data Pad 10, the number of UNSC ships at the start of the UNSC-Covenant war can be calculated:

1.95*10^12 Seconds = 6.2*10^4 years for ONE ship to glass Earth. (That's 62,000 years)

Devide those 62,000 years by the 30.38 year it actually would take to glass the planet, and you'll have your amount of ships. 6.2*10^4/30.38 = 2035 Covenant ships to glass Earth in 30.38 year.

Because the A.I.'s said they based the number of Covenant ships on the number of UNSC ships, this number would be also count for the amount of UNSC ships.

So the UNSC had 2035 ships at the start of the UNSC - Covenant war. Supposedly these are just the warships, as the A.I.'s only calculated the Covenant ships to be warships (CCS cruiser to be more precise).

External

 * GamesRadar: Halo: Reach - Data Pad location guide