The 1st US Congress had 29 senators and 66 representatives (95 distinct people), not all simultaneously, while the 5th US Congress had 45 senators and 118 representatives (162 distinct people, as Andrew Jackson served as both a representative and a senator during this period)[0].

Of the 95 members of the 1st US Congress, 21 (~22%) of them were members of the 5th US Congress, with the following breakdown:

* 5 senators (~17%) remained senators;

* 9 representatives (~14%) remained representatives; and

* 7 representatives (~11%) became senators.

Notes: John Brown was initially a representative of Virginia, then later became a senator of Kentucky (which was part of Virginia at the time of the 1st US Congress). William Smith (MD; 1st US House), William L. Smith (SC; 1st US House), and William Smith (SC; 5th US House) were all different people.

Method: moderately careful transcription plus grep, perl, sort, uniq, and comm.

EDIT: Thanks for adventured for the correction. I mistranscribed Philip Schuyler's name when copying down the members of the 5th US Congress.

[0] I didn't independently check the lists on Wikipedia[1][2].

[1] https://en.wikipedia.org/wiki/1st_United_States_Congress

[2] https://en.wikipedia.org/wiki/5th_United_States_Congress