FIBA's Migration 2020 report gives information about the top leagues (page 27). Other sources are to be found on some league's websites in their statues. In cases we couldn't find the official ruling of the league, we checked the average of senior players with foreign flags on teams on eurobasket.com.
The top leagues and some medium leagues all have either no limitations or at least 6 imports per team. Those include Liga ACB, VTB, Serie A, BBL, Pro A, A1, ABA, LKL, Euromillions, Est/Lat, ABA2, BIBL, Erste MNE,
5 imports is a rare limitation which officially occurs only in the BSL in Turkey and LPB Portugal. The other league that we have no clear definition in was the BSL in Austria, where it seems like teams can have 5 imports on the floor but not all teams have as many foreign pro players.
The most common number of imports in Europe is between 3 and 4, including limitations on the floor or a fixed number of locals that have to be on the court at all times.
These leagues are: EBL, Korisliiga, SBL SUI, BBL UK, A Div. HUN, Basketligan, LNB Pro B, Liga Nat. ROM, Ligaen, LEB Gold, Serie A2, DBL, NBL BUL, SBL SVK, OPAP, ProA GER, Dominos, Superleague GEO, 1st Div. FIN, Total League, BLNO, NM1, KLS, Premijer Liga, Liga Nova, Div 1 BIH, Regionalliga, Nationale 2, B2L, Superleague KOS, Superleague ALB, Div I ICE, Prva Liga MAK, Winner League
Only 2 medium ranked leagues (NBL CZE, Superleague UKR) have a limitation for only 2 imports. The rest of the leagues is ranked rather low.
A2 (no imports), TBL, ProB GER, LEB Silver, Nat. League ISR, NLB, Superleague RUS, Top Div. BEL, Premier League BLR, NM2, TB2L (no imports), Serie B (no imports), Liga 1 ROM (no imports), 1 Liga POL (no imports), Super League IRL, Div B1 CYP (no imports), 1st Div DEN, B-Div HUN, Proliga POR, Div Nat MOL (no imports), DIV A BUL, 1. Liga CZE, SBC SCO, Superleague 2 RUS, Promot Div. (no imports), LBL 2 LAT (no imports), 1. Liiga EST, NBL ENG, Superet SWE, 1. Div NOR (no imports)